You need to do a couple of steps for this, first you need to create a new custom field of type single select list, you can call it the same as the old one but I do suggest giving it a unique name so next steps are not confusing.
After creating the list field and adding the desired options, add it to the current screen scheme the project is using, normally the create screen and the edit / view screen are the same, you will need to see if they are different so you can add the field to the needed screens.
After this is done, you need to search for all existing issues with the old field not empty, export the list to a spreadsheet and then decide which values are going to be mapped to the new values if they are not the same, once you get the mapping done, do a search for each value to be changed and do a bulk edit, here you will change each of the values once, from old field to new list field, you will do this bulk change for each value that needs to be changed.
Once all bulk changes are done, you can go ahead and remove the old field from the screens so users will no longer have it and use only the new list field.
Remember that you will need a re-index after this change.
Take an XML backup before this change just in case.
Let me know if you have any other question.
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