I've been with Orange for 18 years and have always enjoyed excellent service. When I phone them the phrase "oh you've been with us quite some time" often comes up - so I suspect that I often get enhanced offers at renewal due to the effect my contract has on retention figures.

If I migrate to Double E, will my length of service drop to zero and as such lose the customer service perks of a long-term customer?

No, your loyalty will still be recognised. Mine was. I got 10% off when I was on Orange (17 years) as I stayed on the same plan for a number of different phones/contract periods.

EE transferred this over to my EE4G account.
There was a glitch where it wasn't on the first bill so I just called up EE and complained and they transferred it over.

Just make sure you ask before you transfer so you have some ammunition is there's a problem.

They aren't going to say no, they want people to migrate.

I'm not sure loyalty stands for anything these days

it doesn't, after 18 years, they couldn't care less i wanted to leave, so i've gone,Their once excellent customer service is now appalling.