Will updating the carrier settings (UK TESCO/O2) lock a possibly unofficially locked iPhone?

I'm a bit scared to update the carrier settings on my *secondhand* iPhone 4. It is on one of O2's partner carriers (which O2 in itself is a partner carrier with telefonica); TESCO Mobile. This is why I am scared to update:

1. Tesco mobile are prone to lock users to their network
2. There was a guy on applecare who said that apple never locks unlocked iPhones. (Oh, and we fought over antennagate, because it's an iPhone 4.)
3. I have already gotten the 'Latest Carrier Settings' and yet iOS 7.0.6 (or whatever) and iTunes still bug me to update to the 'Latest Carrier Settings'.

I am scared to update mainly because I BOUGHT the (unlocked) iPhone secondhand, therefore, the iPhone 4 could've been locked unofficially by a 3rd party, etc. I don't want my iPhone locked, because it HAS to do me for years to come (even though it's not the latest and it lags when you scroll) and I don't have the money to get it unlocked OFFICIALLY OR UNOFFICIALLY.

Please help me get my head around on whether I should update or not, I just want to stop this message from coming up and from my finger accidentally touching the 'Update' button and it permalocking my iPhone and me having to may more than what the phone is worth, just to use it on a different network.

PLEASE HELP ME!

Thanks!