Example: Aramon Treb
Extract the aranull.fbi file from data.hpi by using hpi view.
Rename the aranull.fbi file to aratre.fbi. Then just place that file in units and unitscb folders.
Don't do anything inside; don't even rename unitname = aranull. Leave everything how it is. That aranull file makes the treb dissapear from the game.
If you removed it, and want to add it to the acolyte for example, you will need to extract 3 files from v3rocket/ipdata/data.hpi files.
Add the aratre pic to your anims mod folder, and rename it to aratreb. Do the same for the script and .fbi file. Inside the fbi file, you only need to rename: unitname = ARATRE to ARATREB.
You will also need to rename the aratre.tdf files in canbuild and canbuildcb to aratreb.tdf, and add it to arapriest(aco) so the aco can build the new treb. The aranull made the old aratre disappeared from all the building units that were allowed to build the treb.
Subject: How to Remove and Add the Same Unit 
