How do you know there is upgrade inventory available? If you are searching for award tickets, that is not the same inventory that upgrades come from.
If upgrade inventory is available, and this is a Delta-operated flight you are booking through Delta, then yes, you can upgrade from a Y fare down through a K fare internationally. Upgrade rates have dropped... 30k for Y/B/M, and 60k for the cheaper fare classes to either Europe or Northern Asia. Anything below K is not eligible though.
So in short, if the agent tells you upgrade inventory is available, but you are not eligible when buying those fare classes, the agent is wrong. If the agent is telling you upgrade inventory simply isn't available, that's another matter.