Why do you need an fdroid version? You can get the straight version from the repo. There's no practical difference.
Discussion
Simplex is in one F-Droid Repo and in your own Repo.. But the are not compatible...
Typica, you can't directly update one variant to another. The update will fail. For example, if you're using the F-Droid variant, you need to back up your database, uninstall the F-Droid version, install the straight version, then import your database backup.