thats correct but to get the new supplier to resync the line and at least try and get a better speed best to start with a clean line instead of just migrating over and keeping same speed.
No, you've got it wrong. The £25 charge only applies if it's a cease (i.e you're not moving to another ISP be it another IPStream supplier or a LLU one).
Asking your new gaining provider to get target noise reset / line retrained is nothing to do with BTBroadband it's the BT
Wholesale department that deals with that stuff.