We have added the ability to do a quick change of events when a person is checking in on race day on the check-in screen. This makes it simple to switch a person from the 10K to the 5K for example.
This also allows you to decide whether to issue a new bib number or let them keep the same bib number.
If your timer is using The Race Director or RunScore, then those changes can be automatically downloaded for proper scoring in the right event.