Agreed. There's no way that a K fare SIN-NRT would be combinable with a W fare from NRT-YYC -- and, even if it were, it might well be more expensive than the W through fare. Note that you'll need W inventory on all flights (including NH804, which I'm a little surprised to see under a native code instead of a UA codeshare).

It does look like a fairly pricy buyup either way, though. Looking at the current fare table, K fares start as low as C$315 half-RT, whereas the lowest W fare I see is C$793 half-RT, and it could be more depending upon the day of travel, original date of purchase, etc. So, you could be looking at ~C$500 in fare difference plus a C$300 change fee if the agent doesn't waive it (per ticket). If you explain that you want to change to a W to use GPUs, that might help induce the agent to waive the fee.