I've looked everywhere... I don't see it explained anywhere! HOW do I change existing unit graphics in Civ4 to ones features in 'Unit Graphics'?

The same way you make a new unit. Just replacing the old... There are tutorials in the Tutorials Section.

Try this (http://forums.civfanatics.com/showthread.php?t=176460). Should get you what you want. Note that's for Vanilla or Warlords.

It's even simpler if you have BtS.

Wolfy, I love you!