Fibre Migration, what actually happens? (BTW to BTW)

I'm due to migrate today from BT Infinity to Plusnet (so both BT Wholesale).

What actually happens? Will my HG612 resync? Will my PPPoE session be forcefully dropped? Does an engineer get involved at the exchange/cab or is it all done in software automatically?

I'm at work but keeping teamviewer open waiting to change the PPPoE details in my router, but the thought occurs to me that when it happens I'm going to lose connection entirely...

might just be a computing database change, but you also may get rewired to a diff vP in exchange, I wouldnt expect modem to resync or DLM reset. Unless new isp is using different DLM profile or different openreach product spec.