1) Tesco use PPPoA.
2) IP addresses are dynamic.
3) You can migrate/regrade speed at the same time, especially as Tesco use something called Datastream. Lines are usually set to run at the highest speed possible, most of the time 2Mbit, and the speeds will be controlled by Tesco / ntl
Bif the process you are talking about is a cease / reprovide, the whole point of migration is that it doesn't involve any downtime, you get code from one provider and pass it to another and the process runs from there.