How do you convert a Choice metadata column to a Managed Metadata column

Hi

We have a document library that currently uses a called 'Suppliers' with various supplier names in a drop down.  This metadata column is of a type 'Choice'.  We currently add to this 'Choice' list as a new supplier is inserted into our business systems.  However, we now have another document library that also wants to be able to insert the supplier name as part of the metadata from the Choice column.  So in my wisdom I thought it would be better to now create a term store that could be used as a single source of suppliers throughout our SharePoint system.  My question is that I don't appear to be able to amend the original Suppliers 'Choice' column to 'Managed Metadata' as it isn't now available in the type list.  'Managed Metadata' only seems to show when creating a new column.  Is this right?  Can I convert an existing column from a Choice type to a Managed Metadata type?

Thanks

Sara