As a rule of thumb: don’t do downgrade. It’s usually not tested, and not safe.
It may happen very rarely, when we stop and rollback rollout. And it’s always scary.
During normal operation your node shouldn’t go back to 99 from 101. (rollout uses the nodeid to return with the same version all the time, until the seed is changed)
In this particular case: I assume 99 won’t see the new style of trash folders, they will remain there. I am not sure if migration to new structure will work, if the process tries to migrate again, and new directories are already created. But may be…