As we've seen, Three can change the terms but in doing so it would also have to immediately give the customer the option to leave the contract early.
It has of course chosen NOT to do this, or else it could get everyone off the One Plan even quicker - but given all those people who might have signed up for two years in March this year, it would be financial suicide to give up over a year of revenue that will be helping pay back a handset subsidy.
Nevertheless, Three could do it and it wouldn't be breach of contract as long as it let you leave.
I really do think a lot of people have lost faith/trust in Three now and are of the belief that they wouldn't be surprised what they did next. Even if Three isn't going to do anything radical, like ditching AYCE data or scrapping the AYCE add-on on PAYG next year, people might think they will and that could sway them towards signing up or buying a SIM card on another network instead.
How mad that a network that scored so many points over the other nets increasing charges in line with RPI, and about Three offering free roaming in many countries, has now ended up having people just thinking 'they're as bad as the rest of them'.
Maybe it's a cultural thing, where the bosses in China don't fully appreciate the impact of their decision. I've seen it many times over the years. I've seen it with the arrogant views of companies like Huawei that expect they can manage the media the same way as they do 'at home' and get rather surprised and put out when the Western media doesn't just say what it's told to say.
I've seen Samsung, LG and HTC all try and do the same. They just expect that they can do whatever they want to do without resistance, but I think Three UK has always been good at making sure HQ knew how things work in the UK (and Europe in general) so as to avoid these situations. But now new rules have been sent down that are going to ruin everything!