Military practice is to promote and then move the newly promoted guy to another unit. Experience shows that it's generally very difficult for guys to make the transition and for the troops to accept it. Fresh start is better.

Temporary boss is a difficult spot to be in because orgs may or may not mean it. I was a temp boss for awhile but that was only until the unit was greatly rearranged and the 2nd level guy stayed scarce to avoid combat with his peers.