Yes, write to everyone to say the terms of the contract were changing - and give 30 days to accept or leave. Or even perhaps longer than 30 days, as there's no reason they couldn't have been more generous.
Three told a trade paper that it had given customers 18 months notice, which is crazy. No customer got told this. Three appears to think that when it discontinued the One Plan and other plans in 2014, this was giving notice to everyone? I don't think so. Ofcom certainly wouldn't think so, as customers need to be told directly.
EE changes its plans next week. Does that mean everyone should consider themselves on notice? As it turns out, EE will let people stay on whatever plan they were on since launch day. Thus far, Orange and T-Mobile users can also stay on their respective plans.
I'd have removed tethering, increased line rental in line with inflation and stated that it will continue to do so in line with new plans, and left it at that. People would have been a lot less upset, and some might still decide to upgrade to get a new phone, or go up to a higher plan for the unlimited minutes, over 2,000 on the One Plan and a lot less on the £12.90 plan.
The point is, nobody could have been too upset by that.
Three has been very arrogant and come up with some crazy excuses. Excuses that nobody has bought, and with the #makeitright hashtag hijacked by angry customers. It makes me wonder if they can honestly continue to let a muppet sell their services in the future. If Three wants to stop being the customer champion and be more like EE or Vodafone, it will have to change its whole way of thinking and operating.
FWIW, TalkTalk lost between 99,000 and 101,000 customers over the last hack, where angry customers just left in anger. I wonder if we'll ever find out the numbers that left Three? There was an abnormally large number of people who said they were leaving just to protest, and others who saw it as a 'sign' to cross to EE even if potentially paying more.