might be a charge from Plusnet for taking on the LLU voice line - check that.
Number portability shouldn't be a problem, but no personal experience.
Going Fibre CP to Fibre CP has a hefty migration charge from Openreach, so clarify if that gets passed on too.
Are you doing it because of monthly cost ?
MaxDSL - goes as fast as it can and doesn't read the line checker first.