The 'upgrade' is available to all if your ISP supports it, and your BT exchange is capable. I am a long term BT customer but had to apply to be upgraded. It does seem that most people get what they want from the upgrade, like spanners above, but there are also many like me totally disatisfied. I was told to expect "up to 3.5 Mb" and have so far achieved, briefly, 1.9. currently stuck at 240 Kb which is much worse than my 1 Mb which was rock solid and reliable.
The biggest problem I have at the moment is finding a telephone number that allows me to talk to someone about this.