Iphone unlock and jailbreaking?

I bought a unlocked brand new iphone and the guy told me not to update it or it would render the phone useless by ridding it of the unlock whih i already knew, but i want to jailbreak it as well to get the most out of the phone so if i jailbreak the phone will that mess it up or am i gonna be safe?

If you jailbreak the phone your unlock should stay safe. But you can also go on eBay and find permanent unlocking services cheap lower than $20. One great service to get your phone unlocked is unlock_fusion on eBay just look them up and its a fast soothing process. Lasts only 24 hours.

Updating should not change the 'unlocked' status on the iPhone, unless it was not done properly. Unlocking is something that your carrier can set for your phone, then you just restore it, and you are good to go to use any carrier. It has nothing to do with software, unless it was forced with software and not approved.
-Jailbreaking is the act of putting non-Apple Software on the iPhone. You can never be sure what will happen when you do that, unless you yourself created the software you are putting on it. It can do anything, including ruin the phone, forcing you to restore it back to factory settings with Apple software, which is not always possible. Again, because most people put other's products on their phones, not knowing what it does or does not do.

Tread at your own risk.