I think this is a no-no, but can I buy a cash economy fare, upgrade with miles to PE then use the Credit Card upgrade voucher to take it to Upper (assuming there's space available?)
You can do that with points so it may be allowed - worth asking and seeing if one of the agents will do it. Pretty sure the Ts&Cs explicitly say one cabin upgrade but it is a bit vague about the technicalities of that.
But you really shouldn’t do this anyway. It’s going to cost an obscene amount of points and you’re going to pay a TON of taxes and fees to do it. It’s unlikely to be the best value for points.
If you want to use the voucher, buy a premium fare and then use the voucher. It’s simple to do and you can often get a decent deal for it.
Honestly, I can’t stress enough that going from Y to J using points is quite often a really bad deal.
One final tip regarding those vouchers, the companion use case offers more theoretical value than the upgrade use case does.
In case you don’t know… The voucher can be a companion voucher, upgrade voucher (return or two one ways), or clubhouse voucher. You pick what you want when you redeem it.