it would be useful for the time being if - in the comments of the posts that will inevitably still be made here - it could be added a crosspost to the new community. Or a bot automatic message.
I actually feel this is gonna make it harder for people to find out about the change, having something suddenly disappear from your feed is less visible than having reminders when you click on a new post.
At the same time, cosidering the .org one is new, not a lot of servers/instances will have the community federated, so it will not appear even in the All feed for people, especially in the small ones.
For those reasons I would advice for a transition period, if possible, but I can see how it could be annoying to manage.
We could set the scheduled posts so that those left behind have the chance every week (or other time period) by a reminder. Just brainstorming. Anyway, this is sticky, that should suffice. I prefer the latter.