I don't know why the highest tier powders can be used to upgrade to T6 again. Usually I accidently end up using a T6, simply because I thought it was a T5. This just seems like unneccessary functionality.
It would make sense for changing 3 powders of one type to another, but then you realise you need at least one of the other powder types to get a 25% chance of getting it back, so yeah just remove this.