I didn't bother with changing the pattern in the end, as the drape of this fabric is different from the drape of the muslin fabric, and with a body shaper it works fine. Looking at these pictures I do wonder if I should not, in fact, change it to a size 8 in the bust area. Still, it looks good and I'm very happy with it!
I even got compliments from Mr T!
Squeezing the pieces into a 1.20m piece of fabric was a bit of a challenge, but the feel of this black knit is great! It's a viscose/elastane combo. Also, brownie points for being completely opaque. Less brownie points for not having any black tread and having the seams in every color in earth. The hem & armhole seams are teal, which I think works well with black. Still, I might revisit this when I get around to going to a notions store.
This dress shall henceforth be named "My Swiss Dress".