This summary is not the rule. The rule text controls.
Here's what this rule means in plain terms:
Before a trial reaches a verdict, the court can allow changes to be made to the formal charges against someone (whether that's an indictment, information, complaint, or citation). The court can only allow this if two things are true: the change doesn't add a new charge or turn it into a different crime, and the change doesn't hurt the defendant's basic rights to a fair process.
If the court does allow a change like this, and the defendant needs more time to deal with it, the judge has to give them that extra time. This means someone won't be caught off guard by a last-minute change without a chance to prepare their defense.
The basic idea is that small fixes to paperwork or technical details can happen during a case, but prosecutors can't use this rule to sneak in a completely new charge or unfairly surprise the defendant. If a change is allowed, the defense gets the time it needs to respond properly.