Thanks. That means I can now conclude that it works for every Copa-operated flight except group reservations, even after splitting the record.

FYI in case anyone ends up in the same scenario: after splitting a group reservation, I had to be transferred to the "redemption desk" to use an upgrade certificate. And not every agent knew I needed to be transferred - some just said no upgrade space is available and one agent said the case will be "escalated" but I never heard back in the suggested 48-hour timeframe. Key is to call and redemption desk will get it done on the spot.