It sounds like what you want to do is pattern grading.  You’re taking a particular size and grading it up to another size altogether.  Unfortunately, we don’t have a tutorial on pattern grading, but we do hope to have one in the future.  In the meantime, check out this article I found regarding how to do pattern grading on commercial patterns.  http://www.threadsmagazine.com/item/4368/making-sense-of-pattern-grading  I hope this helps and you’re able to transform your vintage pattern into something you can wear.  🙂