Far from being an expert on sharing, a few months ago I had to purge
some tabs from shared workbooks, and the Delete option was greyed out.
I had to unshare the workbooks before I could Delete the tabs, then set
the workbooks to shared again.

I assume there are certain tasks that you can't perform on a shared
workbook, since the results could be unpredictable if others were using
them. I wasn't able to find a definitive list of tasks that are
affected by this, but I guess you will have to be prepared to Unshare /
Share when changing the format of a workbook.