Why are people not allowed a complete change of mind? If I have an opinion, but then find evidence that totally contradicts my prior opinion, surely I should be allowed a 'complete' change of mind?

If we're only ever allowed to change our minds by a little bit we'd all be struck in the neighbourhood of whatever (silly) opinions we had as five year olds.

Also, who's in charge of deciding what constitutes a 'complete' mind change vs the apparently-ok minor mind change?