Possible ways to migrate Poster 1 to Poster 2 and a list of incompatible stacks and themes.
If you are using the markdown folder integration, you are directly able to re-use these files inside Poster 2.
For Stacks based items, there is no automatic migration available because of several reasons:
Again: Poster Items from version 1 cannot be used in version 2.
Still it is possible to run both Poster 1 and Poster 2 inside the same project, but not on the same page. So having 2 different pages inside RapidWeaver, one with the original posts of Poster 1, and one newly setup with Poster 2, will allow you to move content from Poster 1 to Poster 2 by copy and paste.
After the new Poster 2 blog is ready, copy the whole Poster 2 Stack to the original page and remove the old Poster 1 Stack. As the URL structure remains the same, all already indexed blog posts will be able to be found afterwards under the same link, as long as title or slug is the same.