Which tones can you switch between?
Formal, casual, friendly, confident, apologetic, direct, enthusiastic and diplomatic, plus anything you describe in your own words. “Make it sound like I am not angry” is a valid instruction, and usually a more accurate one than picking from a list.
What stays the same?
The facts, the ask and the outcome. If your message sets a Friday deadline, every tone still sets a Friday deadline; it just arrives differently. That is the line between changing your tone and changing your mind.
The most common use: sending something you drafted angry
Write the version you actually mean, then run it through the tone changer before you send. You keep the substance of the complaint and lose the phrasing you would regret. This works far more reliably than trying to write calm from a standing start.
Formal does not mean better
Over-formalising is the most common misuse. A message that reads like a legal notice to a colleague you speak to daily creates distance you did not intend. Match the tone to the relationship, not to the seriousness of the topic.