To me, the word "upgrade" automatically implies an element of risk. ie: you MIGHT not be able to. [:0]
Although E.G.G.'s suggestion is a very good one, from the tone of your original post, I'm assuming this trip is already booked and paid for?
From my perspective (and not meaning to sound hard, as I appreciate you're trying to do this as a surprise), if you KNOW you want to sit in Upper Class, you buy an Upper Class ticket to begin with.
If you're willing to add a little bit of risk (and an upgrade to J on a 14-config aircraft is certainly a risk), then you go the upgrade route. This added risk is unfortunately a fact of life when it comes to airline travel. When you've sat with the other 50 people trying to get their "confirmed upgrades" processed on an International UA flight with only 5 seats in C left, you'll appreciate that upgrades are not a certain bet

.
There are only two ways to guarantee a seat in Upper:
1. Pay for an Upper Class ticket in the first place
2. Use Flying Club miles to either upgrade or "pay" for a seat at the time of booking (providing there's space, of course, and this won't work on VH tickets as we know)
Having said that -
Decker's suggestion is a good avenue to go down at this point. If you know there's availability in Z (Which is what VH book into), you could always call VH with this information and ask them to speak to VS directly.