Disclaimer: used to work in SIGINT, so please treat anything I say about this with appropriate skepticism.

There are people that for various reasons do cycle out their SIM card frequently as a means to avoid tracking. This is ineffective. Changing the IMEI/discarding devices entirely is more effective.

But if you change IMEI and use the same SIM card, you're still trackable as if nothing changed, right? The IMSI would be the same. I guess you need to change both at the same time...