I believe the best approach would be to incorporate an easy migration option within the Google Play version of Amethyst.
This could involve adding a "Migration" tab to the sidebar, which would provide brief instructions and download shortcuts for the migration process. You can also use popups to encourage users.
In github version you should also include shortcut to install latest version of amethyst from github.
And I think you should also start subscription model for monetization.
Everything is just my personal opinion, please do what you think is best.