It would need to reinstall - which violates current ostree policy. Any change to that policy would need to be requested upstream based on my current understanding. If I have that wrong, I am hoping one of the maintainers will correct me.
As the policy is defined, after initial install, /etc and /var are handled with special, and differing rules. /etc undergoes a 3-way merge but /var is not touched after install.
Rollback from the grub menu or reinstall are the best current options. And if /var is borked - reinstall is all that will currently help aside from restoring a backup.
FYI - if such a feature was made available I would like it to be opt-in. I have been bit hard a few times over the decades by things like that. I would reinstall anyway. I would not trust the integrity of the result. Because any problem that is likely to surface will not do so until months later - maybe during the next version upgrade. It just isn’t worth it to me.
I wrote an article here to summarize the behavior. I learned a lot while writing it!