Out of interest, how did you upgrade?
I think the short answer is that downgrading is *possible* but could well not succeed. Did you not make a system snapshot/backup/image before upgrading? (if not, next time, I would highly recommend that you do).
For more info on downgrading, search the web for "ubuntu downgrade", e.g.
https://linuxconfig.org/how-to-downg...evious-version and
https://askubuntu.com/questions/4986...evious-version
I personally would do a reinstall if I found myself in that situation.
