I'm considering purchasing a Flexi fare NRT-SIN-CPH that is eligible for upgrade with KF miles.

Want to confirm one thing about the SQ upgrade program prior to booking. Looking at the SQ.com upgrade chart, the cost of a one way saver upgrade from Zone 7 (NRT) to Zone 11 (CPH) is 55,000 miles. I did an award search for saver business class on both routes (assuming that is an indication of upgrade space), NRT-SIN is showing available and SIN-CPH showing waitlist. Do both segments need be "available" in order to select an upgrade or can I upgrade one and waitlist for the next? Also, will the cost be identical if one segment is upgraded vs both?

Thanks