1. If you turn a denim jacket into a pair of denim shorts, you should indicate in the describe/worn that this is some kind of 'upcycled' fashion, for example 'This pair of denim shorts looks like it has been remade out of some other garment, pieced apart and then sewn back together.'
2. You should be mindful of fabric. You shouldn't turn a pair of navy blue denim jeans into a pair of slinky navy blue leggings, even though denim and lycra might both have started as 'synthetic cotton'. Similarly, if you buy a vinyl jacket which is described as mostly black but with @color neon green accents, you shouldn't turn that into a neon green bathing suit.
3. You should be mindful of size. If you buy a gold silk g-string, you shouldn't be turning it into a floral patterned gold silk kimono.
In general, I was told that RP should trump code where tailor-play is concerned, with I think is the general sentiment in 'help tailor-play.'
Obviously, there's no way to code-check for these things, but I think there should be a section in help tailor-play about it -- I've heard recently a couple conversations about people discussing retailoring where what's been OOCly expressed is 'if the code allows it, we can do it.'
This is the language I would add to 'help tailor-play':
Respect prior tailoring: If you remake an existing garment into something completely new, make sure that what you are doing makes RP sense. Describe your new item as something recycled or upcycled. Don't change fabric from denim jeans to stretchy leggins, even if both are codedly synthetic cotton. Don't turn a tiny g-string into a full-length robe. Someone who sees both the item before and after you alter it should say 'yes, those changes make RP sense.'
I think that would clarify that RP trumps code where tailoring is concerned, prevent some abuses in terms of market shopping, and also encourage more 'upcycled' or 'recycled' fashions which are pretty CP as far as the Mix is concerned.