It's likely that's the ONLY way that you would have any chance of successfully upgrading, as P+ pax have priority on the waitlist. For Silver in December, attempting to upgrade from economy is a virtually certain fail. Whether you'd have any reasonable chance at all from P+ depends on the particular flight. But in general the odds are not high, particularly for four pax.

Note the warning above about being unable to unwind your Chase transfer if the upgrades fail.