Hey guys, so recently I acquired an old used Samsung Galaxy SII, AT&T model i777

Since I live in Europe, I had to get it unlocked, so I went to a shop, gave them the phone and they unlocked it for me.
Now, the guy at the store said, if I update the Firmware it will be locked again - is that true?
It's currently Android 2.3.6 and I kinda want to have it on the newest one..

Also, I am bit worried he might've put unwanted software onto the phone, the shop didn't seem particularly... Trustworthy... (since unlocking here is kinda an under-the-hand-deal). Are there any ways to check?

Can you help me out with that?
Thank you in advance