Q: How do I use D:

Click on your old save file as normal. Use "open modlist from save" to make sure the save is clean. Once reloaded, add this mod, and choose "load anyway." Now you can convert all your leathers. Remember to sell off any sub-group leather apparel, or recuycle them if you prefer (if you have a mod that does that) and convert those too. Once satisfied, save your game, disable this mod, an enable Simple:Chains Leather

Q: But won't people exploit this mod to convert leather in regular games? D:

Cheesing leather is not the intended function, but people will probably still want to use the mod for that. And that's fine, Rimworld is a single player game, they can do what they like. Besides, they can only cheese the core leathers, and most other leather are of higher value, so it would be situationally beneficial.

Q: Why did you make this mod? D:

I have a colony that pre-dates my interest in expanded processing, and I'm just glad I will be able to play it again without seeing a ton of those Red X errors, or losing out of the truckload of leather supplies in that colony's factories XD

Q: How long does it take? D:

Crafting items takes ages, but converting the leather is a quick process. I made the mod coz I did not want to have to craft a zillion items and sell them before I could play my old save :P