Sorry, I should have used more precise language. I'll go edit my initial post after I type this.
The system doesn't actually cancel and rebook. Rather, any changes requested have to meet whatever rate and/or stay restrictions that are currently in place. If everything is copacetic, the change will go through and the reservation number will remain unchanged.
Example: Original reservation is for Friday arrival for three nights. Imagine that the guest wants to amend it to arrive on Saturday for one night. If that Saturday night currently has minimum stay requirement of 2 or more nights, that change will be denied.
However, if the guest wants to change to arrive on Saturday but still stay three nights, the change will go through with the same reservation number. And anything connected to the reservations (like payments) will remain.
So, it's not really that reservation changes are actually canceled and rebooked. Rather, it's just that any changes must meet the current restrictions in place.