I've done it several times recently via chat for itineraries that got changed by JetBlue so I couldn't modify them on the website myself. I had paid with points and they charged it as a new seat fee and refunded the prior one. They said it would've been identical process if I had paid by credit card.

Mosaic members should do this on any 2022 flights they booked EMS seats on in 2021 now that it's only 100-200 points. I've had over 13,000 points credited for prior bookings. This year EMS points will be refunded after you fly, but why have all those points tied up?