So my switch to Plusnet was completed yesterday and I can confirm that my line has undergone some sort of reset. It has gone back to syncing at the full speed.
current linestatus
state: UP
mode: VDSL2
mod type: Profile 17a
snr margin down: 76
snr margin up: 148
latn down: 143
latn up: 140
satn down: 0
satn up: 0
output power down: 19
output power up: 141
rate down: 79987
rate up: 19999
attainable rate down: 86232000
attainable rate up: 23044000
chantype: fast
IP profile has gone back to where it should be.
IP Profile for your line is - 77.42 Mbps
The line resynced at 6 AM yesterday, at which point, the access circuit to BT was removed. During that resync, the line jumped back to this new speed. Took the best part of the day for the Plusnet service to come online.
I can't imagine that a BT engineer was out at that time so all the changes appeared to have been carried out remotely.
So far so good with Plusnet. Internet access is just as snappy as it was with BT. Plus bonus static IP! Haven't noticed the effect of their traffic management yet, but still early days.