Newly promoted staff have a tightrope to walk. If they were promoted from within, they may have friends in the department. A positive manager will support the newly promoted person to transition into the new job. If not, seeds can be planted for future conflict.
If the newly promoted staff person comes from outside the company, the person has to acclimate to the new company culture, rules and regulations, and learn the players. A positive manager will support the newly promoted person to transition into the new job by assigning an ally who is well positioned to minimize the learning curve. The manager can take the person to lunch and privately share insights. Also, a formal introduction to the people the person is most likely to have contact and s/he will have interaction.
It’s helpful for the manager to understand the new person from that person’s perspective. Also, have the new person lead a staff meeting and give others a chance to exchange with the person.