Is it possible to change the backoffice provider myself afterwards?

Alfen charging stations are "simlock-free".

This means that Alfen charging stations can be connected to many different backoffice systems as long as that the back office supports versions 1.5 (NG-Platform only), 1.6 and 2.0.1 of the OCPP-JSON protocol.
You can also switch backoffice suppliers at a later date, for example by purchasing a 'standalone' charging station (without a back office) and configuring it at a later date to connect to a backoffice system of your choice.

For detailed instructions, see the Knowledge Base article How do I change the back office of a charging station?

Note:
The charging station is simlock free, however, most Chargepoint Operators/Backoffice systems restrict mobile access only via their own provided simcard