In Debbie Stoller's book 'Stitch & [censored] Nation', she really explains not only HOW to read a pattern, but that upon understanding, how to modify it the way you want.
If you think you've already got the understanding a pattern part figured out, then, on page 10 of the book, she has a chapter called 'Gauge of Reason - replacing one yarn with another of the same gauge' and on page 13, she has a chapter called 'Gauging a Reaction - knitting a pattern using a different gauge yarn'.
I highly recommend just getting the book yourself, and using it. She has some pretty neat ideas, and helpful hints, that it's well worth the $$.
I'm not in any way affiliated with her or the book - I'm just a new knitter, with no one to teach me anything, so I use her book alot !!
Good luck, and let us know if you ever figure it out.
April