The ambidextrous magazine release was a bad design and caused many failures in the field. Glock isn't exchanging the guns out of the goodness of their reputed heart.
When I bought my G21SF, I didn't know there was an issue. I was relieved to find out that mine didn't have the ambi mag release.
I would work toward getting it replaced.
Google around, it seems that Glock pays shipping both ways if you call them first.
"No matter how cynical you become, it's never enough to keep up." - Lily Tomlin