" 6 of 1 and half dozen of the other". A classic case of the unclear and mercky guidance affecting listed buildings across the board.
There appears to be lot of room to retrospectively " change the goal posts" after a c*%k up.
Seems simple to me, they should change the windows but be given a reasonable timescale to do it in as it is very costly to do it all at once. At least it's an issue in Llandudno. In Birmingham I'm certain anyone can do whatever they want to a listed building including demolish it with pretty much no repercussions.
I agree it seems simple, but I somehow doubt that the offenders are going to quietly agree. They'll likely make a big case about how the council is forcing them to change perfectly good energy-efficient windows for less efficient ones that will massively increase their fuel bills, all at great cost to their businesses at a time of economic depression and probably forcing them all out of business. Exaggeration? Probably, but that's what people do when fighting their corner.
The report also suggests that there are many seafront listed buildings with uPVC windows so they will be used as precedents, forcing the council to bring them into the battle as well.
Of course, the really sad thing is that whatever the outcome, our built heritage is already the loser because the original windows have already been lost.