It's not a rolling upgrade, but it allows you to move the apps, PVs, policies in as granular a manner as you wish.
Because of that major change, the cluster upgrade path is a little more involved than usual. It's a complete reinstall of the OS and rebuild of the cluster. There are tools to help tho, and as the path is tread things will get easier and better supported. If you go through the same wave as me, you'll be annoyed at first but then once it is done and you have a 4.x cluster you'll be really happy with it (especially when you can manage everything as an operator).
Luckily from an app perspective very little will change since Kubernetes is the API. A sibling comment linked to some helpful documentation. I can't be specific right now but I can tell you that your need is known, and some very smart people are working on it. If you want to email me (check my HN bio page) or jump in the Keybase group called "openshift_okd," I'm happy to chat more about it (not in an official Red Hat support capacity, just as friend to friend :-) ). I haven't done a migration myself yet but I know people who have, and I plan to get into it personally soon as well.