Why didn't you buy it unlocked at 1st place?!
Anyway, update it without worries.
If you have a locked device which Apple has unlocked, it is counted as already unlocked device. But don't expect too much from iOS5.
At June 7, I got this email:
http://i51.tinypic.com/iep76a.pngAnd of course one of the working device was updated and ... "That's all of it?!" The big excitement turned into a big disappointment.
My suggestion: Instead of to iOS 5, update to AND 4. The just-out-of-the-oven Galaxy Nexus is quite suitable for that. That's what I am going to do.