If the database is not too large, i suggest you do export/import for both stacks. This is a supported and easy way to do it.
Otherwise your procedure might need some manual tweaking, it might be possible to try something called dual stack split. You will be able to export the java stack and you can then try to reimport it to the abap stack. But you most probably will have to intervene at certain points to get that working.
Cheers Michael