Are you trying to change multiple posts are once, or just one at a time? The latter can be done by just going to your edit post page in the admin panel, and changing the author there.
If you’re trying to do bulk, use the bulk actions drop-down at the top of the edit post listing page.
Yup, it seemed to be having the old version of the sideblog plugin that caused it. Upgrading to the latest version of the plugin solved the problem.
Thanks. 🙂
