Availability of all subgroups of J fare, for direct sales, decides the main part of the difference you have to pay. If discounted J fares are all gone, you need to pay the difference between full J and your econ ticket price.

Purchased subgroup of Y fare, decides any add on price, and also, if you are allowed to rebook into another fare category at all.

I bought in Y once, and wanted to pay up to a PEY ticket. Availability is abundant, but was told by CX that for that sector, up-sale from Y into PEY is not allowed.

It would depend on which other airline you are talking about to know if it's easier with them. But I'd say QR or SQ will most likely be happier if you want to pay them.