To add to @Kurt_Jones here - if you want to get rid of the old field:
1. Create a new report with a column for the current/old field and a column with the new field. Of course new field column will show blank. Decide how far back you need to keep this data and you could just filter in the date range in the report.
2. Transfer the existing info for each from old column to new column. Yes this is manual but depending how many tasks/projects you have (whatever the object is), coupled with how far back you want to go, it’s not that bad.
3. When all info is transferred that you want, you can safely remove the old field.
I did this when I went from a text field to a dropdown field and really didn’t want the old field lingering around in field sections even if only visible to admins.
@beccashinnick
If this helped you, please mark correct to help others : )