Getting upgrades, I can only mention with QF in mind, you dont on an international tix get guaranteed upgrades, on the lower tiers, ie, PS/SG, even if you have noted your interest by applying for them.
If you are WP/P1/CL (google the meaning of the codes as mentioned by JessicaTam, your chances of an upgrade using points are better (international travel) than PS/SG.
On a domestic tix, its easy to use points to upgrade, if there is space, you only pay for the taxes due, and thats it, the upgrade is yours.
On an international tix, its a raffle. No guarantees of an upgrade, not till you get to the airport, so you cant use the QP. If you have enough points for a QF J ticket under points plus pay, that is the best way to get a J seat.
I would not use an upgrade using points, because you do loose out of gaining SC.
If you use points plus pay, say, I have got 31,000 QF FF points, and I want to get from SYD to AKL in F class, on a QF flight number, flying EK (Emirates), I would use my 31,000 QF FF points, pay about $990 or so, and still get the QF FF miles earn, and also 120 QF SCs.
I only complicate matters, but maybe someone can explain it more succintly, ... but points plus pay to get into F where they are available, or J is better than using all points for an upgrade, ie you have a Y tix, and want J, and use points for the J, not good.
Instead pay more for the J tix, and then use QF FF points to reduce the cost of that J tix.
Edit: while I am at it, if you read on QFs website about top up points, I would not do that either. You will on top up points, or using points upgrade, earn SC at the fare paid, ie, lower earn rate, at Y fare level.
But, if you use points plus pay, you will earn (most important), you will earn SC at the upper earn level, ie, if you paid by card and points plus pay, for a J seat, or F seat where available, that is what you earn SC at, the uppear J/F level.
I am going to hide under the bushel now, I am sure to get scolded soon!
Sorry for the waffle.