I read that mobile network providers can block IMEI numbers from most networks but I also read that criminals can change these IMEI numbers in order to regain access to a network. How is this done and what can I do to protect myself from it?
criminals can change these IMEI numbers in order to regain access to a network.
Not only criminals, changing IMEI is easy even without root access, there are plethora of apps that can do just that.
How is this done and what can I do to protect myself from it?
For most devices, IMEI is stored in EEPROM, simply raising a voltage from normal working one will allow you to change whatever is stored on it.
You mostly can't do anything to protect only IMEI. Hardware limitations. However, you can for example encrypt your phone internal storage, set a startup code, enable device locator and so on. This will make it harder for criminal who stole your device to actually profit from it.
Unfortunately, not all phone models support that (startup encryption)