Some of us agonized over how long it was taking to be the Manager. Others had the desire and drive but were surprised when the time finally arrived. But my guess is virtually none of us had any training for the transition from peer to supervisor.
The list of don’ts and bad decisions is limitless. Don’t suddenly become a baseball fan with the one in the corner cubical. Don’t pretend to know why someone else wasn’t selected. Hopefully the person showed pretty good interpersonal skills to warrant the promotion, and can be more vigilant and deliberate in exercising them. Assume less. Ask and listen more.
But I think the most important thing is to simply talk with each of the ex-peers. Most will need some kind of assurance. Those who feel they deserved the promotion need to hear a strategy for making them promotable in the future. The buddies need to hear your heartfelt concern about their relationship evolving.
My real trial by fire though, was being hired as the Manager and the staff weren’t told until I walked in the door. Can’t see myself ever forgetting that day. A couple thought they were still being considered. To my advantage, as an outsider, at least they couldn’t say I wasn’t qualified.