This page contains a table of what the old services were & what the new ones are.
ALthough they are still very vague about exactly what level of data transfer you can have, it is sort of unlimited in that you only get into trouble if your affecting others services.
I think.
Upgrading from lower speeds to the 2mb when available can cost £14.99 if your not prepared to wait for the automatic update once BT have finished fettling.