I find this to be unlikely, Unity makes a significant amount of effort to make upgrades as painless and seamless as possible, its the whole bases of their business model, to make game development easier and more streamlined. I'm not saying that upgrading unity versions isn't without its issues...
They have mentioned it in a few different videos they been wanting to wrap up 7DTD to do another zombie game but without the building/destruction aspect because it will give them more freedom to do different things. I think evidence points to that's what they have already started, they have more...
Ugh! hope they make the game as moddable as they were saying... Already want to reverse like half the changes they have made to Alpha 17. Also sucks they are spending so much time on other projects that 7DTD is getting left behind...