Wondering: is there any way to change the lichcraft spell...

[Edi]

It might be doable if you can find a way to combine some of the spell effects of Gift of Reason (targeting another unit), Transformation (shapechange) and the demilich's unit ID as the target shape for transformation.

Good luck trying to get that cocktail to work.

Supposing that you did get it to work, the unit would have the base stats of a demilich, but it would retain its own magic, experience etc. If you wanted increased Death magic, you would need the relevant effect from Twiceborn, which increases (or should increase) D magic.