So you want to cut off the sleeves/upper body part and keep the lower body, and do sleeves starting from the top of the ribcage, going up to do the sleeves again? That should be very easy if the jumper is knitted in stocking stitch. Research 'using a lifeline' and more importantly, a 'destination row'. I believe there is a video here showing how to insert a needle to make a destination row.
Basically you put a needle through the fabric, through the stitches, all in the same row (further back in your knitting), and then you can cut the knitting a row above, unravel that row and you will end up with the lower row on the needles, ready to start again. Then take the upper body part and unravel them from the cut row up to reuse the wool. Is this what you mean? I'm not entirely clear on what construction you used for the jumper.