How do I move 30 rows of knitting to a longer circular needle?

Just knit from one to the other.

I have only one set that I change the size by screwing on different lengths. What then?

Oh! Can you slip the knitting onto the new cord and then attach the ends?
I don’t have interchangeables, so this is off the top of my head. If you don’t think that would work, you could put the knitting on waste yarn–run it through with a tapestry needle–and put it on hold while you change the needle.

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.

I understand. I’m off to try it. Thanks!