You need to standby on the original coach ticket's fare class. Once you've cleared that list, and make it on to the earlier flight you can try to upgrade.

So you aren't standingy by for X-class upgrade inventory, If your original tickte is K, L, Q, or whatever, that's the class you need first. Only after that do you worry about X-class inventory.

In general, it's fairly unlikely that you'll clear the upgrade on standby.