Yes but you said about reversing it Im saying about the unused bit of Eccleston's bit from just after it started changing to Tennant.
They would only need his head from the shot and past it onto Hurt's as Hurt's Doctor changed.
It would need a lot of clearing up but it wouldve been better than a reversal.

Well now matter how they could have done it, and they certainly could have done it if they'd wanted to, they didn't!
Why we'll probably never know until Steven Moffat writes his version of The Writer's Tale, and maybe not even then!
It certainly wasn't for any technical reason, as Moffat has tried to imply. That's obviously complete nonsense!

Why is it obviously complete nonsense? Sure, you can do pretty much anything you want if you've the time and budget nowadays, but that doesn't necessarily mean they had the time and budget to do this shot justice.

the fact that you say moffat is now trying to imply it was a technical issue, just makes me think that in reality, he just misjudged the situation. He obviously thought at the time that seeing the face at the end of the regeneration (as we have for every single other regeneration), was somehow not important, then after the episode has had complaint's and found himself having to come up with excuses to justify it ever since

I think you have hit the nail on the head so to speak!

With all the other huge CGI elements in that episode, it beggars belief that they wouldn't have had the time or the money to take that regeneration shot a couple of seconds further if they'd wanted to.
For Moffat to imply otherwise is just not credible.
In fact didn't he imply somewhere that they made the full transition but didn't use all of it because they weren't happy with the quality?
That's even more unbelievable I'm afraid!
No, it was a purely editorial decision, for whatever reason.

Why is it unbelievable that the quality might not have been good enough, though? You understand that they have a limited budget and have to decide which FX shots deserve the most time and money on? Taking it a few seconds further doesn't mean it would have come out looking convincing.

Yes, it would be simple to provide a static morph to a photo of Chris Eccelston. But there's no guarantee that would end up looking convincing. In fact, I'm surprised people are even arguing the point. The very little we did have looked poor to me as it was. I remember that final shot of Hurt smiling, with the energy pouring over his face and thinking "this doesn't look nearly as good as previous regen effects. What's going on?"

Now if Eccelston had been involved in the filming, I'm sure it would have been much easier (and cheaper) to pull of the regeneration. But he wasn't, so the only options they had were to use a static image, to edit in some existing footage of him or to layer his likeness onto an animated 3D model. Of those options;

A static image would look totally unconvincing. You could pull off the morph, but it would be blindingly obvious it was a static image. Look at the multi-Doctor shot at the end of the episode for proof. They can just about pull that off because it is a long shot and they have three of the Doctors who are actually real. But study the individual other Doctors. They don't look convincing. Imagine that up close with just Eccelston's face. IT would not look good.

Existing footage might work, but equally, it could end up looking unconvincing, as it was never intended for such a scene. Particularly, if it had to be reversed, it would be noticeably weird, as reversed footage is usually quite obvious by perculiar motion.

A 3D model with his likeness overlaid could work quite effectively for a short shot. However, this would be quite a time-consuming operation and most likely far beyond what would have been available for such a short sequence.

You can't look at the FX work form the rest of the episode and then use that as proof that the FX on the regen must have been good enough. All that means is that they spent more time and money on building sequences like the fall of Arcadia. Ultimately, they might have felt that plot-important events deserved more budget than a few seconds of regen sequence - especially when doing the regen justice might have required far more work than the length of the scene deserved.

To assume Moffat just didn't think the scene was important is ridiculous. If he thought that then, one, he wouldn't have written it in the first place and two, he would have cut the entire sequence out and ended Hurt's Doctor with him simply leaving in the TARDIS.

Its actually far more convincing that everyone was telling the truth and that the FX were created and just weren't good enough to make it into the final episode.

Presumably even more so given that they didn't seem to be able to afford a full regeneration effect for Christmas, even with both actors on hand?

A CGI regeneration sequence is relatively easy and cheap to do compared with a full scale space battle scene.
I just don't buy that they couldn't do it well enough for it to be included, and I certainly can't believe it was because they spent the whole effects budget on the other sequences!
They must have had plenty of face shots of Christopher Eccleston that they could have used from his episodes to do a perfectly adequate regeneration sequence.
It would only have needed his face on screen for half a second or so, and not even clear of the regeneration effect.
There should have been no need to use still frames or reversed footage.
Sorry but I can't be convinced that it was anything other than a purely editorial decision not to show anything of Eccleson's Doctor.

How do you square that with the absence of the effect at Christmas?

But we're not talking about simply doing a CG morph between two accurately-framed shots. We're talking about framing John Hurt as they would like and then morphing him in to stock footage. That might be easy and cheap to do, but it isn't easy and cheap to do convincingly.

Just look at John Hurt's face in the final shot of the regen scene. It seems quite clear he is trying to keep completely still. In fact, I wouldn't be surprised if it was actually a still shot of him and they were doing this specifically so they could line up a particular still of Chris Eccelston to his face. It looks weird in the bit we do get to see. I'd put good money that it looked even weirder when you see the completed shot.

It makes no sense that they would just decide they don't want to show anything of Eccelston. They used stock footage of him for the freezing of Gallifrey and they even made his appearance stand out more than any of the other "stock footage Doctors." so they obviously didn't have some issue with showing him in the episode.

Then there's the fact that Moffat bothered to write the regen scene at all, then send instructions to the FX team to create a regen shot from Hurt to Eccelston. If it wasn't wanted, they wouldn't have bothered at all. If they decided they wanted to drop the regen at the last moment, they could have cut the entire regen scene out and had Hurt just fly away the same as Tennant.

There's absolutely nothing to suggest that there is a good reason for them to not want to show Eccelston. None at all. Their official explanation does make sense, though. It suggests a good reason as to why they wouldn't want to show the full sequence. The idea that the sequence looked perfectly fine, but that there was some mysterious editorial reason to chop it off before the end just doesn't make any sense.

If the scene was perfectly fine, then why chop it part-way through? If you decided you wanted to edit out the regeneration, then why not edit the whole thing out? It makes zero sense.

I don't, but I suspect it was simply that they wanted to do something different!

Sorry I still don't buy it!
I worked as a TV post production technician until I retired, and while I can't claim to have been directly involved with CGI myself, i worked closely with many people who were, and that transformation could have been made perfectly convincing, even with footage of Eccleston lifted from somewhere else.
I just can't be convinced that it wasn't an editorial decision. They wanted to show the regeneration to get closure on John Hurt's Doctor, to remove any speculation that what we saw wasn't the end of his incarnation. It is possible that they did want to allow speculation that it wasn't Eccleston's Doctor that he regenerated into, but I think it's far more likely that what my friend told me was true, that using Eccleston's likeness in that context was just too problematical for legal reasons so they decided to not actually show his face clearly, and let people just assume that it would have been him.
We could argue about this forever and probably never agree!
I hope one day that the whole thing will be cleared up properly, perhaps when Moffat finally stands down and writes his inevitable book about it!