I have an existing IQ2 panel in my home, it looks like the original owners broke their Brinks contract out of term and Brinks will not unlock the panel for me. Is it possible to swap out the SIM with a GC2 SIM card and work with Alarm.com?
No, you cannot swap the SIM card in any Alarm.com module. Doing so will lock down the SIM and make the module unusable.
Is the module definitely tied to an Alarm.com account still? You can check here.
If Brinks will not cancel the account currently tied to the module then you would need to replace the panel.