A regular DB can always be dumped into MySQL... actually anope never uses only MySQL, it always uses it .db files as well.
Migrating from 1.8 to 1.9 will always be made possible, but the reverse it not. If you use 1.9, you will not be able to go back to 1.8.
MySQL will be added to 1.9 at some point, but it is not yet on the TODO list, so it likely does not have a high priority and other things like redoing the regular database will get priority.