Unfortunately some of their nodes do seem to have problems with speeds more than others.
Good news about the retrain though
A straight forward migration doesnt usually involve a retrain.. and any line data such as the MSR, FTR and other line profiles stay the same.
However a full retrain does occur if going from adsl1 to adsl2+
Expect your line to go through another retrain when moving to an adsl1 ISP from adsl2+