Does upgrading iOS change my IMEI?

Does upgrading iOS change my IMEI? I was told not by AT&T not to remove my SIM card or certain restrictions that they had placed on the iPhone, at my request, would be lifted. I want to know if upgrading my iOS is like removing the SIM card. I think this was because the IMEI number changes when the SIM card is removed from the phone.

Does upgrading iOS change this number?

The IMEI is kind of like a MAC address for the cellular network only. It is nearly impossible to change it, you can only spoof it on a software level. So the answer is, No, your IMEI will never change unless you really really want it to. Removing the SIM card shouldn't change the IMEI either...

The IMEI shouldn’t change when you upgrade the phone, at least here in Europe. Mine is still the same.

Update: Upon some reading around, it turns out that if you “hack” your phone, (maybe in the past), an update would change some IMEIs, and AT&T would therefore invalidate the SIM. Be careful with what you do :)