@bobjonkman  you are right. Leaving the old group and being only a memeber of the new group was enough to be able to post to the new group.

Once there was a problem: people couldn't join quitter.se-hosted groups like \!tea, \!wordpress. When I checked from a remote account I noticed that that instance still tried to connect to the former Identica \!tea group, even though the proper "http://quitter.se/group/tea" URL had been pasted into the ostatussub text box. Lets hope we won't run into that problem.