From what I've read (and actually also I am a little bit nervous about this), the major problem is that this is NOT seen as an actual merge.
From how this change was presented, this is more like a "reset and start from scratch", NOT a merge.
A merge is where a server merges into an already evolved server, where players can continue their development on an already developed world.
But here, it seems that we have to just throw away all these years and years of development and planing, and start from 0. Of course, we don't start from 0 with towns, but with islands, with the world itself. And this will have a HUGE impact.
For example, in my case, since I am playing this game for over 16 years, for some reason I lost my early donations. My donation score is a joke, even though I've donated a lot in the early weeks of the game. The mines where I am settled now have these levels because of donations of other players that do NOT exist, at the moment. They just quit the game, but I continued here.
Basically, we should start on a new world, with donations, but you don't keep in mind that in our current world we have donations that are just vanished. Even if they are not OUR donations, it is part of the game we played until now, and should NOT be erased.
I don't know how the new world would look like, I am not against a merge, but I do not like the idea of starting from scratch, with level 0 mines. At least, copy the progress of a current world (with regards to the mines levels), and put us all there.
At least, this is my opinion on how this merge should be addressed.