I would take the mostly-basted sandwich over to my machine and stitch on an extra bit of fabric to the backing to make it longer. you don't have to undo or unroll anything as long as you have the space to maneuver the rolls. Then once that is stitched on, you will have a long enough backing and you can finish basting.
Of course you could not do that and just trim it to size when you are done, but that depends on how much it will upset you in the end.
I would be hesitant to un-baste it all, being that you would stretch and distort the fabric, rip the batting, loosen seams and threads, etc.