So, I copied the building and acolyte sacrifice ability. But how can I make my version convert my custom unit A into custom unit B? It doesnt have an unit required - unit result field.

Just took a look at Sacrifice. Those fields are conspicuously missing...

My guess is that your "from" unit is any unit with the Sacrifice (Acolyte) ability, and you "to" unit will always be a Shade, no way to change it. Worst case scenario, the "from" will only work with the standard Acolyte unit. If that's the case, you may want to change your Custom Unit A so that it is a modified Acolyte, not a new custom unit.

As for the replacement unit, I'd suggest using triggers to replace the Shade with your unit, and another trigger to simulate rally-points. The limitation here is that you'd only be able to use a single rally-point, not the queued rally-points created when holding down the shift key.

I do hope there's a simpler solution that I missed, but you may be out of luck.