Is it permissible for a girl's guardian to annul her first engagement and accept a second one who is religiously and morally better, and would this be considered proposing over another's proposal, which is Islamically forbidden?

A Muslim proposing to a woman already engaged to another Muslim is forbidden if the second suitor knows about the first proposal. However, if he does not know, there is no sin upon him due to ignorance. It is disliked for the woman or her guardian to break off an engagement unless there is a benefit that necessitates it, such as disliking the suitor, in which case she has the right to protect herself.