SIM card has nothing to do with it.
If the handset is branded (if it was sim locked, chances are its branded too), when the carrier whose branding is on the device releases an update, you will get it, regardless of what sim card is in the device.
To unbrand and get OTA updates in future, you would need a gold card. Since your gold card is your SD card that is in your phone, if you lose it, chances are you've lost your phone too
Also, not sure about the IMEI thing. Changing an IMEI on a phone in the UK is illegal. If unlocking a phone from its sim lock changed its IMEI (which it doesnt) then that also would be illegal in the uk, yet it is not.
Also, its worth noting that unlocking and unbranding a mobile phone are 2 different things.
Unbranding (using a generic rom) would give you a generic software build number, which then would receive updates as soon as HTC release them, rather than waiting for the carrier to do it. You could then not get an update from the carrier.