They are of seperate booking classes.
If there is availability in C for Business and A for First, you're most likely able to upgrade.
The fare buckets for reward are N in economy, I in Business, and X in First if i'm not mistaken but these do not appear through seatcounter and the like.

If you're unsure about the upgrade availability call Skywards. Sometimes the online system does not show availability whereas in reality there is (I did it many times). And they are very helpful and willing to burn your miles for an upgrade (unlike some airlines that make you feel you are trying to steal from them).