If the ID is already in use, you will have to modify it directly in the database. Of course this will not be officially supported. If you don't want to mess up with your database, create a new attribute/enumeration, replicate the value to the new attribute and abandon the old one.

There are process XML based enumerations, there are database based enumerations. They might work different.

Anyway, what you could try - and I would suggest to do that on a test system first:

1. Add a new Enumeration literal to replace the broken one
2. Change all work item values from the broken literal to the new literal (use API or CSV import/export)
3. Remove the broken literal.

I don't know if this would work or not. I am pretty sure it would work with database based enumerations, I am not so sure about the Process XML based enumerations.