I've just gone through the whole lot, since all the episodes are now on 4OD, and I'm blowed if I can detect any of this 'character changing' referred to by Linehan.
My impression, quite to the contrary, is that the characterisation remains pretty consistent throughout all 3 seasons, with an irascible Bernard, an amiable Manny and a slightly detached Fran.
IMHO the same can be said pretty much for the writing, despite changes of writer.
Maybe I'm missing something!
'Black Books' is a series killed off way before its time. When you consider how other sitcoms have been flogged to death, this one by comparison still had huge mileage in the tank when it stopped.
I can see no reason at all why it can't be resurrected. At a nonsensical 3 seasons, it comes preciously close to doing a 'Fawlty Towers' on us.