Hi Nick, it's not quite clear entity or field you are referring to, but there is no easy way to change the data type of a field from it's existing type to an option set (aka a dropdown). You'll need to create the new field and add it to the form(s), then change every dependent script, workflow, plugin, and view to refer to the new field in place of the old field. Then you'll be able to delete the old field.
There is an add-on that can help automate some of these steps if you need to do a lot of this type of work:
www.wavextend.com/.../Customization-Assistant. I haven't tried it, but you might want to evaluate it.