I did the very same thing a few years ago with a police trade-in 17. They even gave me all new mags. (I had shown up with three mags). They aso asked if I wanted to upgrade to extended mag release and slide catch. So, I said sure. 20 minutes later I was holding my gun again, still warm from function testing.
I live about 3 minutes from the Glock location in Smyrna. I was also suprised at the way my G20 was returned to me. I purchased night sights anger basically got that plus everything swapped out for new. I only had one mag on me, the rep told me to go home and get the rest of my 10rd mags, and he replaced them with 15rd mags.
The only thing they will not replace is any aftermarket parts.