What happens if I unlock my phone?

Dear AndroidCentral,

What happens if I Sim Unlock my phone? That would mean that I would be able to use sim cards from most GSM carriers. I have the ATT galaxy s3, with the stock ICS. But, if I unlock it, and go to Tmobile, would I be able to use it? I know that each carrier has it's own version of Android(i.e. ATT jb, Tmobile jb, etc). Would I be able to use a Tmobile simcard, on my s3 with ATT software if I unlock it?

You can use it on T-Mobile once you unlocked the SGS3 AT&T. However, depending on where you live and if you have data or not, it might be EDGE (2G) instead of HSPA (3G/4G)

I have one question. If my phone was locked and then I unlocked it wil I still get OTA software updates?