How about an answer to this question (timely, in light of the current CR-1 promotion):
If I confirm an upgrade using 15,000 miles, will I be allowed to switch to using a CR-1 at check in instead?
In theory, no. In practice, sometimes yes.
The process of changing upgrade instruments isn't a simple swap-out. In the case you describe, there would be technically four steps: (1) downgrade into Y (assuming there was still the applicable Y fare you originally purchased), (2) put RDMs back into your account, (3) reapply new instrument (i.e., CR-1), (4) be reinserted into the DM upgrade queue. If there was enough NC/NF inventory, it wouldn't be a problem; you'd be back into the upgraded cabin supported by the CR-1 instead of the miles. But if there were a waitlist already in place at check-in,
technically you'd have to be reinserted into the list based on status/fare/checkin time. Unknowledgeable agents would take some time to go through these four steps; those worth their salt could do it quickly.
Your question, of course, is whether you could "hold" the confirmed upgrade with RDMs, then swap out for the short-duration CR-1 that you'd earn
after booking but
before travel. The short answer is no, you can't do this.
That said, if the agent is good, and inclined to help (i.e., break the rules), it is possible to create a dummy booking in the upgraded cabin (even if it is sold to capacity), move the details over to the new booking, then deduct the CR-1, without having to put you back into Y first. They have to be good (and fast) to do this, though, and it certainly can't be counted on.
So, the end answer is: Don't count on it.