We were trying to change an existing Managed Metadata Site Column from allowing single values to allow multiple values. When we did so, we received this error:
/Lists/Projects : Cannot change this column to allow multiple values because it is
currently being indexed.
The error is a little bit misleading, as it might lead you to believe that the list is currently being indexed by search. (At least, that was what my client and I took it to mean.) In fact, the issue is that the column is indexed in the indicated list, which in this case was: /Lists/Projects.
I found a Microsoft KB article (2015261) which explained that “This itself is expected behavior.” The article applies to Microsoft Office SharePoint Portal Server 2003, but the same process fixes the issue in SharePoint 2010 (and therefore, I would expect in SharePoint 2007 as well).
By manually removing the index on the column in the Projects list, we were able to set the Site Column to allow multiple values. Note that multi-select columns in a list cannot be indexed in the list, so there’s a trade off. However, I’ve never had anyone explain clearly what difference adding an index to a list column gives us in the first place.