Swap skins for the custom unit.
Import the skin twice: first for the custom unit, and then rename the second for the original with the import path.
Skin for Custom Unit: Alpha Shaman (eventual Orc Warlock).blp
Skin for Original Unit: units\Creeps\ChaosWarlockGreen\ChaosGreenWarlock.blp
Even better, don't bother swapping skins for the Custom Unit. Instead, use the import path at the bottom of the first named skin I provided you.