Sukraj is correct. You can't convert some field types to others, but String (UTF-8) is convertible to String.

Just look up Tier ID (probably named u_tier_id) in the sys_dictionary table where table is core_company, and change the type from String (Full UTF-8) to String. Save the dictionary record, and voila, you should be good to go.

If you do want to go with deleting the field, it would probably be a good idea to run a background script first to copy the values out of the old field into the new field; otherwise you might have some data loss. Reply if you need help with that, if you go that route.

Hope this helps,
--Dennis R