I did what?
 

U10 tournament. I'm on the bases and call several IPs (only the very obvious ones), some which negated a called strike, as you might expect. The coach was fine with it, his team was rolling. However ... my partner tells me after the game that the fans were perplexed and annoyed because, on the called strikes that were changed to a ball because of an IP, they had never seen a base umpire overturn/overrule his partner's strike call.

Fans are almost as ignorant of the rules as coaches. :p

There's a difference between overrulling a call, and changing a call due to a rule infraction, which is what happened here. No different than when a BU calls a stealing runner out, and the PU has to change it due to batter interference. Or the PU calls a runner out at home, and the BU has to change it due to obstruction. Or....

Or, because an IP becomes a ball automatically at the time of the infraction.

Perplexed fans at 10U? The wonder of it all! :D

What was your partner's point?

They still haven't! :)