You can upgrade the databases and load pre-12.5 database dumps and transaction logs.
Some issues of which you should be aware:
Upgrading requires space for copying data and logging changes to the system tables during the upgrade process. If the source database in the dump was full, the upgrade process might fail. While this is expected to be uncommon, you can use alter database to extend the free space in the event of insufficient-space errors.
After reloading an older dump, run sp_checkreswords from the new installation on the loaded database to check for reserved words.