I think you need to clarify this a little.

Post the existing table def, and what you want the table def to be after the alteration.

If lots of foreign keys link to that table - its going to be messy

But whatever the eventual method take backups, test it out in a safe non-production environment until its working, and it may be advisable to do the production run in downtime or single user mode in case of locking/changes during your change.