You might or might not have to change your string and cables. There's no way for anyone to know without knowing the exact model and year of your bow, what cam and stuff it's got now and what draw length you need.

You can go to Hoyt's website and look up the specs for your bow, see what it has now and find out what cam, string and cables you'll wind up with at your new draw length. Look under the 'Technical' menu and find 'Tuning Charts.' http://www.hoyt.com

I don't have a clue what it'd cost to do all that, but I'd guess it'll put a big dent in a $200 bill. Maybe more. But, like you said, it's not as painful as $600. I just hope your bow is a recent enough model they'll have the replacement cams for it.