It's not a software flash for the handset, it's an unlock code from the phone manufacturer so the handset im 99.9% certain will be perfectly fine for you to do whatever the hell you want with it
Just be warned, if your handset is super new manufacturing date the codes are almost impossible to come by right now. I've now tried several places online and local stores. I've just given the premium, super new unlock service mentioned above a go and will post results here.
HTCsimunlock actually worked out around £15 with the processing fee of a couple of pounds, when my phone came back blank for a code they wanted another $15 and another paypal processing fee to do a "deep search"...no thanks.