Migration from Photon cloud (PUN) to Photon server

I have a small multiplayer game built using Photon cloud that we would like to migrate to Photon Server to take advantage of the additional features available with that architecture.

The game is already live with a small but sizeable user base which means and I am trying to understand how best to go about migrating the app.

Has anyone attempted this before, and if so, how should I go about doing the same?

Thanks