Solution:
It turns out that the Move-SPUser
command just goes trough all people columns and replaces the old username by the new one. Newsfeeds (MicroFeed list on team sites/project sites) store the information about persons who posted in a column PostAuthor
which is (don't ask me why Microsoft did this!) not a people column but a single line of text column. So the Move-SPUser
does not affect this column and it has to be changed manually in any way. I think a PowerShell script would be the tool of choice to do this.
Edit: Have had contact to Microsoft SA Support regarding this case. They do not recommend changing user names. They say it would be better creating new users instead of changing user name. This means lots of work (NTFS permissions, distribution groups, sharepoint permissions, user profiles on end devices...).