Couple of points to note:
D class cannot be upgraded using points. Very attractive D fares ex DEL but they won't get you into Suites.
SIN-FRA-JFK is an extremely popular route and two upgrades (if at all) would only happen for Advantage Rates (148,000 miles per person)
If you book A class, you pay 2,250 USD per leg to upfare from U to A each way.
My suggestion: Book A outbound and U return and you know you fly Suites for sure.