Last year I purchased a ex-TPE discounted Y ticket using online channel, then later up-fare it to discounted J by calling eService Center

I was charged the fare difference between and 2 receipt
1. Original Y Ticket
2. Fare difference payable in HKD (as eService Center is based in HK)

I see 3 options for you to move forward with this ...

1. Find a TA, get a payment receipt for the Y portion and pay out of your pocket for fare difference
2. Call CX, explain you want to buy a Y ticket that can be rebook into PY later
3. Book Y, cancel the ticket (and use that receipt for expense), book PY again

As per CX Website, most discounted Y can be rebook with a penalty so (2) should work without any issue https://www.cathaypacific.com/cx/en_...arketcode=USWW