Hello,
In this case, you will need pre-defined array of values which is implemented CRM Dyamics using Option Sets (either global i.e. common across entities or local to current entity). As this is related to region, it would be better if you go to Global Option Sets as per link mentioned in above post. This will help you carry this same option set to other entities as well if needed.
Once ready with your global option set, you'll need to create a local option set (by using existing global option set) which will map to this global field. Then replace your old srting attribute on form with this option set to make data entry consistent.
For existing records, it will be advisable to create a utility which will update records in bulk as per your new option set & existing values. This can also be done manually if there are less records & technical help is not available.