Change multi select custom field to single select?
I am not sure what would happen if the field contains multiple values.

What I would do if I where you is the following:


Create a new field, exactly as you want.
Export your contacts with only 2 fields: internal id, and the multi-select field
Check the export and fix the records with multiple values
Import again, but then use the new field via import mapping
Check your data
Disable old field (disabling means you can enable again when needed)
After a few months and no need to see the old field -> Delete old field