Hi Jen! There seems to be a couple of things here that may need clarifying; Canvasses may, of course, be used over again, and I often do this myself, but a possible problem here may be that you wish to make a new painting using pallet knives.
How prominent is the present pallet knife painting? Can you apply enough paint to cover any existing marks?
Its theoretically possible to turn the canvas over, but you may end up with cracks this way along the stressed edges, but this may also depend upon the age and quality of the canvas.
As far as coating the canvas with Gesso, I would'nt recommend this personally, as you would be applying a water based product over a (varnished?) oil painting and I agree that a Gesso coating may crack.
You could try an oil based primer, using a couple of coats,sanding down in between to give some tooth.
It may be cheaper and more advisable to to use either a new or alternative canvas.
If this isn't the answer you want, then come back to me and we'll do our best to get what you are looking for.
Good lack and very best wishes,
PA & RSC Wigan / Warrington / St Helens