I am seriously thinking of back tracking and cancelling the whole changeover
NOOOO! Don't do that!!
Have a 1:1 with your manager, though, because this kind of information really must come from as high up as possible. It's likely that it will be announced later, though it seems like there was the perfect opportunity already, he may have something else planned.
Do have a chat with your manager, or even send him an email asking that he mention it to the team as you feel this info is best coming from him and not you. Ask him to just send a quick email to the distro or something, but that he be the one to say it.
If you're not comfortable with asking outright like that, arrange a meeting where you ask him for his advice! Flatter him a bit and ask if he's got any tips for you, anything he expects from you, etc., and then throw in the announcement after that.
You will need to get used to going to him with this kind of thing. It gets easier over time, but when I was first promoted to manager, I never thought I was suited for it, but my manager could see that I was wrong about myself and just lacked confidence. I felt like the most frightened, timid person on earth and although I was good at organising and managing time, the rest was scary. I was extremely conflict averse to the point it felt like a phobia, and the first time I had to tell somebody off, I was sick in the loo after. I still get the shakes when I have to discipline somebody for something really bad, but it gets easier every single day and those really hard parts are few and far between.
My manager was right, and I'm comfortable saying I'm a damn good manager today. I'm certain your manager is right about you, too, or he would not have put you in that position.
Please don't even think about backing out over this one tiny thing! Just take it into your own hands and tell him to do it!
And congratulations! You'll be fabulous, I'm sure of it!