This opt-in migration is already running @Weafyr: Turris OS 4.0 alpha2 is out!
You can already migrate to 4.0 if you like. There are still big buts in there but as we are working on 4.0 more and more documentation and guides are going to be created so it should not be hard to go from 3.x to 4.x.
I was responding on request of updating to 4.0. This is not possible without migration script so only way is to wipe configuration and start from scratch. That does not mean that we are not supporting clean installation, I would say that right to the contrary we do more than updating. I would also note that it is not a good idea to use backups from 3.x release because few important configurations were changed and router can be inaccessible if you load backup from 3.x (at least until configuration migration is done).