I’ve been asked this question often, over the years, and I can tell you a little about my own experiences with older patterns on modern Barbies. However I do not work for Mattel or one of the professional pattern makers. I have been sewing for Barbie since I was a child in the 1970’s, though, and I sew for both vintage and newer Barbies.
I have measured my vintage Barbie’s waist and bust, and compared these measurements to my modern Barbies’ waists and busts.
Vintage Barbie’s waist measures 9 cm (3.5 inches) whereas modern Barbie’s waist is 9.5 cm (not quite 4 inches). Vintage Barbie’s bust measures 15.5 cm (6 inches), whereas modern Barbie’s bust is only 12.5 cm (approx. 5 inches). So there is a slight difference. And when sewing for Barbie, even half a cm makes a big difference in how a garment fits.
With that said, I have a collection of both vintage and modern Barbie patterns, and I’ve never had a problem getting my doll clothes to fit. I think this has something to do with the way I measure darts when fitting my Barbies for their clothes. Just in case you’re new to sewing, let me explain. The darts are the seams that we sew from the waist to the tip of the bust, to allow “give” in the curvy bosom area.
Before sewing darts, I lay the doll on a flat surface, like an ironing board, desk, or table. Then I take a straight pin and pin the darts where I think they probably fit best. I try the bodice on the doll with the darts pinned. If it doesn’t fit, I adjust the pins. Then I mark a line with a fabric pencil (just a regular lead pencil will work, as long as you’re not sewing light-colored fabric), and I follow my pencil line to sew the darts.
This should help you adjust the older patterns to fit newer dolls and vice versa. The trick is to constantly try the clothes on the doll as you go. (I probably do this ten times when creating one garment.) That way you can seam rip, if needed, before you get too many pieces put together.