I was recently informed that I am being promoted. I'd like to know how to effectively go about communicating this to my colleagues before the promotion is made public knowledge so as to ensure that it won't place any strain on my team's dynamic.
Ideally, you don't communicate it but your immediate supervisor does. They can call together a meeting of your team to say something like "THE DOCTOR is going to be doing X. This change has not yet been formally announced, but I wanted to let you know what is happening before you see the announcement. Y will be taking over THE DOCTOR's work on project A. Z will be handling (some other aspect of your current job)..."
I felt it was particularly important to do it that way when I was leaving a team leader role and would have no authority over that team in my new job. I couldn't say how people in the old team should be assigned after I left it. Of course, I had been closely involved planning and advising my manager on how to keep my old team working well.
Since you're staying on the same team, doing similar work, but with added leadership duties, you should allow your management to inform your team, instead of doing it yourself. Doing so yourself would more than likely be seen as presumptuous and overly eager on your part. Imagine how it might be interpreted, "I'm being promoted, and I'm going to be telling you what to do from now on."
You should encourage your manager to inform the team before an official announcement is made.
After you begin your new duties, it will require extra effort on your part to help your team know you're still on their side, while at the same time, having to balance their desires with business priorities. Congratulations, and Good Luck!