Can't help re the socket question, but it does of course affect the migration side of things.
Normally, you just migrate in the same way as ADSLx >> ADSLx, so yes - you need a MAC from O2. I went from O2 LLU to IDNet FTTC. There is a short break in service but I don't really remember how long. You did of course have a similar short break when you moved to O2.
Your connection at the PCP (phone) cabinet has to be linked into the FTTC cabinet, and at the exchange at some point you have to be connected to BT kit for the phone instead of O2 kit. That's because on LLU the phone goes through the LLU ISP's DSLAM even if it isn't LLU phone.
Your broadband connection via the FTTC cabinet could end up anywhere, it often isn't even the same exchange as your phone/ADSLx.
As far as I know you cannot have ADSLx and FTTC on the same line.
My broadband basic info/help site - www.robertos.me.uk
My domains,website and mail hosting - Tsohost
. Internet connection -
IDNet Home Starter Fibre. Live BQM
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.