If you did not start in 9.3 directly and did a previous upgrade from 8 to 9.3, you can create a branch from that revision and open that branches in 8xx. Than you can manually copy the logic, since copy past won’t work (i expect).
It’s not possible to downgrade.
If you don’t have many changes, the best thing may be to check out the last version you had in Mendix 8, and reimplement the new functionality.
Hope this helps.