Since your relapse was very recent, it is unlikely that the roots of your teeth have shifted to such a degree that would be significantly different from your initial finish. I do not think any major harm would come to you by proceeding without a new set of orthodontic records.
I personally repeat the xrays, photographs and moulds that I took pre-treatment when finishing as a dentolegal record and also to assess certain aspects of the case.
Post-treatment xrays are useful to detect root shortening that can occur as a result of orthodontic treatment. If this has occurred, then your re-treatment may need to be more conservative.
Perhaps your orthodontist wants to only take the xrays again at the end of your retreatment to reduce your radiation dose. Since you are currently under the care of this practitioner, I would trust that person to do what is best for your interests. I would suggest that you ask your orthodontist for his/her reasoning if this is an issue that concerns you.
