However I also think it would be contentious for them to make a volume 14 that would include two volumes drawn by Miura and one made without him. Doesn't mean they won't do it, though.
Their options to avoid this would be to include the guidebook or even the Grunbeld novel (which would be worse imo) in either 13 or 14 instead of a volume, so 14 would end on 41, or perhaps include four volumes worth of content in 12 and 13 so 12 would have volumes 34 to 37 and 13 would have 38 to 41. I'm pretty sure they will just have 14 be 40, 41, and Studio Gaga's 42 though.
Deluxe Editions 12 and 13 already say which volumes they're collecting, the usual three, so unless they make a monumental change of course, nothing different is happening before 14, when something different definitely will one way or another.
As we've discussed, concerning the editorial sense, or lack thereof, of these of these collections I would be pleasantly surprised if they did anything other than publish the next three sequential volumes, let alone something clever like doing 40, 41, and the guidebook to add a natural, yet unobtrusive, partition before the continuation. Maybe shocked would be the more apt phrase though, because that would suddenly make them more thoughtful than their Japanese counterparts, AND go against the publishing purpose of NOT renaming or separating it; if you want all of Miura's Berserk in this format, you're also going to buy in to NOT Miura's Berserk, might as well keep going then...
I also suspect this doesn't require a brand new contract negotiation, like with a new or separate property, with Dark Horse or their other publishers around the world; they're simply continuing to publish "Berserk" as is, it's just no longer by the same author. I think the only consideration that would gum up the works is if the new Berserk proves to be far less popular and/or lucrative, and therefore less valuable, than when terms were agreed to.