if I understand you correctly, I think you are trying to replay a text field to dropdown field and wants to deleted the text field.
if yes, You should do in this sequence
1. Create the new "dropdown" field and publish it.
2. Disable the old "text filed" and leat it on the form so no new records created.
3. Ensure the data from old field to new field is migrated
4. Ensure all your data correctly populated in CRM
5. if you have any workflow or plug-ins or any customization running change it to new field and redeploy
6. final remove the old field from form, do not delete it just make sure you workflow or plug-ins does not populate data in this old field any more
like Neil mentioned, there isn't any easy way, but if you follow above steps would be simpler