I'm pretty frustrated by this too. If anyone knows of anywhere with a tutorial or explanation of what needs to be changed to fix an existing default replacement package, I'd love to know. I'm capable of doing a little editing (S4PE, Sims 4 Studio, etc.), but I don't know how to track down the issue itself. So far, I've been Googling all over the web and not found anything that actually says. (There were a couple links that claimed to have this info, but they were broken/didn't exist anymore and so forth.)