Changing a call?

[IamMatt]

By way of introduction, I am new here and appreciate the opportunity to learn. I am a dad of a 12-U DD and as a rule, I have the utmost respect for you officials, who do a difficult job under stressful conditions.

Here's my question:

First batter at my DD's game hits, runs to 1B, defense fields the ball and throws to 1B before the runner hits the base and the PU calls her out.

Then the manager (coaching at first) calls down to the BU, "Her foot was not on the bag!" and the BU says, "I agree. She's safe."

I would think if an ump makes an erroneous call and immediately and spontaneously recognizes it and corrects it himself that would be rare but correct, but if he has already made the call, how kosher is it to change it under the circumstances described?

Thanks.

Matt