Here's another (inexpensive) idea.
If there is upgrade inventory available, buy the miles you need to cover the difference. Since you will need 25,000 total miles, buy 10,000 miles for $350.00. With that, you receive a 75% bonus, giving you an extra 17,000 skymiles. Add that to what you already have and your at a total of 25,000 miles, just enough at your current fare class to get the confirmed RT UG's.
Also, maybe you just buy enough miles to UG one way and fly coach the other. I mean, your looking at a flight time of under 5 hours westbound, and under 4 hours going east. If money is an issue, then buy only enough miles for a one way upgrade. Buy 4,000 miles for $140.00 and your set.
if your willing to take a few moments, get a subscription to
www.expertflyer.com and you can see for yourself if the UG inventory exists or not. Or, list your flight info here and I'll give a looksie.
I just did a quick glance at EF for various dates in April. G class is available on a smattering of dates and flights so your chances are.. decent.