If you have something like the Denises then you will have the stoppers that go on the end of the cables.
Move your stitches up the cable far enough so that you can undo one needle. Take that needle off and put a stopper on that end, (obviously not the end with the working yarn). Put that needle onto the cable of the desired length and put a stopper on the other end of that too.
Now you will basically have two straight needles, one with a cable longer than the other.
Knit from the shorter cabled needle onto the longer cabled needle and when you're done simply take the stopper off the end of the longer cable and replace with the needle from the shorter cable.