Hi, I'm looking at booking a trip from SFO to ICN on Singapore, and I'm hoping to use a SWU to upgrade. I noticed that there are two economy fares available:

Flexi-Saver:
- Earns Miles
- Not upgradeable with miles

Saver:
- Earns Miles
- Upgradeable with miles

While it seems obvious that a flexi-saver can't be upgraded with miles, could I still use a united SWU, or is that out too? The Saver fare is about $70 more.

Thanks!