I have tons of RDMs, so personally I'd use the miles now if the upgrades can be confirmed and save the GUCs to use later or use for flights where the upgrade could go to the gate.

If someone doesn't have excess miles or doesn't travel enough to use the GUCs well later, there's more of an issue, but I don't see a downside to securing the upgrades with miles now and deciding later whether to try to switch to GUCs. I would expect agents to insist on first cancelling the upgrade (which IIRC requires that your original coach fare class be available, so you might have to upfare) and then confirming the GUC supported upgrades from available upgrade inventory at the time. The upgraded seats you temporarily cancel might not go back into upgrade inventory, so attempting to do this can be risky if there aren't multiple seats in upgrade inventory.