If you migrate from one provider to another at the same address there should be only a brief loss of service.
It would only be if you moved that you would face an issue.
I left BT and joined another provider and the transfer was done at 5 am, I didn't even know.
You join the new provider who handles the migration. You don't contact BT to cancel, or this causes more issues.
Its still a cease and re-provide. The cease would release the port to the next Openreach order, but there may not be a port left to do the provide, so the order will fail.
Well that's interesting because I was told my CAB was full and swapped providers and it didn't affect me... so how does that figure?
You were either extremely lucky or although full, there was nobody else waiting for a spare port so you just got your old port back without anybody else diving in.
Another thing to factor in is the wholesale provider, once the connection leaves the exchange then it comes down to the peering/transit agreements of the new provider.
This is why you see people having different speeds with different providers, the backhaul is the bottleneck.
I am aware that different ISPs each have their own backhaul arrangements, and assumed that this is where the cheaper providers make some of their cost savings?(Cheap things are cheap for a reason).
I was more concerned about the potential for my local link dropping from the current 80MBits down to a possible max 15.5MB!