>When applying for using a certain frequency band, you have to specify usage and purpose, and you can't just change your mind, i.e. you can't apply for GSM frequencies and say you will be running a GSM network, but then run the PavelLishinOverAir protocol.
While this is mostly true, in practice (at least on the 450-470 MHz band I work with), you can give fairly vague purposes, like "telemetry" or "SCADA", and it doesn't matter what vendor or what protocol you use. In fact, there is no section of the licence that says "I am going to use protocol X on equipment Y manufactured by vendor Z" or any part thereof (emissions designators are a different story).
I don't have any experience will cellular licences, though. Given that carriers frequently repurpose spectrum as technology changes, I can't imagine their licences say "GSM only" or anything like that. The band that used to be 3G GSM is being used for 4G LTE on my phone right now.
