Well, this isn’t necessarily the case. Depends on the layout.
If you create an image as multipart non resizable off of a fresh cd install of windows 7, the d1p1 and d1p2 files, for all purposes, are rec.img.000 and sys.img.000 respectively. So you could, potentially, switch this image from non-resizable to resizable. However, if you create a resizable image, you are kind of stuck. This is because the mbr is not available.
If you were to go from multipart to resizable, you’d need to rename the d1p1.img and d1p2.img on the server.
Rename d1p1.img to rec.img.000
Rename d1p2.img to sys.img.000
I hope this helps.