How to Play with Barbies has a great tutorial that shows you what the inside of a Barbie head looks like (let's have a minute for the Pop Life Christie that was sacrificed). So to make a regular barbie head fit Rosie, you have to remove part of her throat (the visible underside of her head) and part of the column of rubber inside her head.
This shows Barbie's carved out head sitting on Rosie's neck. This head could probably stand to be shaved down a bit more, and is obviously the wrong color. I also wrapped a bit of duct tape around the neck post to give the head something to grip to.
And of course, here's Rosie's actual new head, the lovely Raquel.
Now go forth and re-head those Rosies!