Mysql backup
An SQL backup will save all your themes, just make sure to save your settings.php and config.php file as well.
If you want to do a fresh install, you could try the merge system, although I don't really know how that works with themes. I believe if you keep one you need to keep the other, although you can always just delete themes if you'd like.
If you want to do a fresh install, you could try the merge system, although I don't really know how that works with themes. I believe if you keep one you need to keep the other, although you can always just delete themes if you'd like.