Hi Jason,
I'd suggest:
(1) keeping existing fields, but hide the question and answer in the view. -- you may need to set section and put question and answer fields to the area to be hidden.
(2) Add new fields for new questions.
(3) Add new field for Question Version, and set V2 or so.
Then
- for new form, hide old question and answer
- for already submitted form with old QA: they should have blank Question Version. so show old questions, and you may hide new questions. when you open submitted form