I'm currently on ADSL2+ with quite a decent monthly discount (I was ready to jump ship).
As the application process seems to be web-based, I can't see a way of finding out if I would retain all/some/any of the discount if I upgrade.
My guess is that if you want Infinity, you pay list price, which would make it unattractive.
Hopefully, I'm wrong!
do it over the phone - Call the customer options team.
I am paying £20 pm (first 3 months £7.50), with free phone calls and line rental for £10 pm.
I upgraded from the adsl2+ 20mb to infinity, just call customer options and they can usually do something for you.
They also are giving out a sainsburys gift card which Im sure they can throw in.
Its best to negotiate, and if you arent successfull just say I dont think I want to upgrade yet and try again later.
Eventually you will find someone who is willing to do a little better.