One cannot switch between Sonos firmware versions. As firmware updates are released the user has the option of refusing the update, but once the update is installed there is no going back. All of the Sonos units in a system must be running the same firmware version and an update must be installed if a user wants to add a recently introduced hardware item. For example, before the recently introduced PLAY:3 can be added, the system firmware must be updated.

I don't think that hardware additions will cause much trouble because the Sonos protocol is open, however, some of the music services and iHeart Radio may need special consideration in a 3rd party controller. Standard functions such as PLAY, PAUSE, STOP, VOLUME, etc. use standard calls and should be stable across firmware releases.

Fischi, I have no experience with the Fabrice controller. The Philips controller is very limited and slow. There may be some latency in your Pronto's WiFi connection. You can minimize this by removing any encryption and limiting the Pronto to your own WiFi channel. Make sure that your Pronto has a strong WiFi signal. Also, experiment with pressing buttons on the Pronto a little slower. If you are an impulsive type who immediately pushes a button (again) if there is no immediate response, the Pronto may be flooded with multiple requests and responses.

The Sonos user forums are a good resource. Someone over there may have some insight.