Position Synch / Manual Update Issues

Content

I set Enterprise HCM with Manager = Use Position Hierarchy

However, I need to override some of these for a few workers.  There is more than one person assigned to a parent position.  
However, I cannot delete the Use Position Hierarchy until tomorrow bc it Position Synch was done today.  
Is there anyway to reverse the position synch?

Also, going forward if position hierarchy is changed, how do you update the managers in bulk if you don't have it set to use hierarchy at the Enterprise level.  Or if you set it, how do you make manual updates where needed?  If I use change manager, I get an error stating only one line manager allowed and there is no way to delete the current LM.