Hi,
Your best option would be to contact SmartBear support and ask if they can do something on the backend for you. I've no idea if they have the ability to do this but if they can't:
Next best option is to use the API as it has an UPDATE endpoint meaning in theory that you should be able to a) create your new custom field, and b) add the text to your new field (cleaning up the old text by using the UPDATE endpoint or maybe deleting the custom field if the UI permits
Next option is more involved and uses the UI features. If you haven't run any test executions on your test cases, you could 1) create your new custom field, 2) export your test cases to CSV, 3) move your data from old to new fields, 4) archive your old test cases, 5) import your amended test cases, 6) re-do the work in getting test cases into test case folders, test cycles, etc. Note: if you have run any test executions, then you're going to want to keep those test results and an export/import isn't going to work.
That's all I can think of right now.