Internal promotion of manager has just been proposed. But here you are, you will have to mentor former colleagues. Here's how to be credible and impose on you in front of employees who do not always accept your evolution.
One of the mistakes often made is to radically change your behavior, either by being too familiar with your teams or by imposing yourself as an authoritarian leader. By behaving in this way, you provoke the rejection of your team. Prefer to stay yourself and do not change your habits overnight.
However, you will have to assume your new duties and exercise rigor, for example by reframing a member of your team who would make you inappropriate remarks because it would be difficult to accept this change of role.
Also, try to avoid cronyism. You were probably close to some people before your new duties, but that should not be felt in the work and you should pay the same attention to everyone.
Determine the new framework
Your former colleagues do not know you as a manager. Try to arrange a presentation meeting where you will announce your new features. Show how you want to work (meetings, reports, project management software, etc.) and do not hesitate to explain to everyone, on individual points, what you expect and what you would like to bring them professionally.
Stay in communication, listening, and exchange as much as possible and take the time to answer each other's questions.
Your promotion may annoy some who already saw you in your place. You will probably be angry and jealous. That's why you'll need to be patient and give your teams time to accept your new functions.
Develop your self-confidence but also that of others. Do not apologize for becoming a chef. In addition, if your journey in the company and your reputation do not speak for you, you will need to give confidence to your teams, for example by being transparent and honest, by encouraging your teams rather than giving them orders, being grateful for their work, etc.
Share your vision, show your leadership and decision-making ability, show your intention to bring the team together around a project, and so on. You can create proximity with your new subordinates by organizing team building events or extra-professional activities.
Do not take anything for granted!
Finally, stay humble and never stop questioning yourself. Management is not an exact science. You have the right to make mistakes, it's all about taking initiatives, trying things and improving yourself continuously.