The last engine change I did 2 years ago, I only notified on the v.5. and the new V5 came back without any questions.
The quote I got from DVLA website appeared about a year ago.
It would appear that persons at DVLA that answer the phone don't know their own rules.
I would guess if you tried to notify an engine change today they WILL ask for the info I quoted.

Regarding the reference to engine size and emissions / tax bracket, that's not the same as emissions as measured at MOT.
People were claiming they had changed their engine to a smaller size to get reduced RFL, when actually they had the original engine in.
DVLA fixed the RFL to original makers emissions data so no matter what engine is in it's still the original RFL rate.

More people were falsely declaring reduced engine size than people genuinely declaring increased engine size, so DVLA's income is hardly affected by the few who fit large engines to cars with low RFL rate.

At M.o.t, emissions are determined by whichever is the oldest, car or engine.

.

.