Can an owner ok a change order, have the GC issue the CO to a sub then later change what he wants done to the work in the CO and because he doesn't like the amount ask the site contractor to complete the work issued to the sub and ask for a credit from the sub?
You need to read the contract documents carefully. There should be a section regarding the owner's right to add and delete work via change order. Based upon those terms, the owner may be able to add scope then descope based upon price if they reserved that right. If they did not, and an actual change order was properly signed, then it is unlikely that the owner can de-scope the change order without being exposed to a claim for lost profits on the unperformed change order value.