I booked a lowest-fare Y return ticket MXP-EWR with TP a few months ago (before the US reopening was announced), as the price was exceptional (sub-300€) and I didn't want to tie in too much cash by going directly for J before knowing whether I'll actually be able to travel. Now that this has been cleared up, I'd like to upgrade to J, but going for a fixed-price upgrade for each individual segment through the TP website would result in a ticket that's significantly more expensive than purchasing a J ticket outright for the same itinerary.

Would a TP call centre agent accept to simply upfare my ticket from Y to J with me paying the fare difference, or am I stuck with the website-offered upgrade options?