You want to move your phone line to get 8mb? I wouldn't bother! I'd upgrade!
Then again...it's better than 2mb I suppose.
I think with BT you're gonna have to phone them up - because it might effect other things - I doubt anybody would be able to tell you what to do, because your hook up depends on a number of factors - and unless you've worked with wiring before, get an engineer - you might be able to get a non-BT engineer and save some money...that said, it's kinda their fault that you're getting 2mb instead of 8, so they should be obliged to do it.
Then again, are you using 8mb to start with? With newer systems, the object is to use as little memory as possible, so it might only require 2mb to do what you need to do - you might not need 8mb.
Again, it depends on a range of things, so hiring an engineer is your best bet.