Edit: Er... Just Realized that you meant the Study Guide Book, and not the entire class... much of this post no longer applies to your situation, but might be some useful information still, since I think the Guide Book is based on the Blue Book. hehe.
No, you really don't need to relearn everything - even if you don't think you remember it now, after doing some basic brushing up most of what you've learned will come back to you easily.
I found Kaplan to be an amazing review tool for the science and QR, they've got online quizzes and workshops that have at least a paragraph explanation for every question, and that's where I did most of my learning - from getting things wrong and finding out why. It's pricey, but in the long run it's worth it. The Kaplan Blue Book you get has just about all of the science you'd ever need to know in more detail than you'll need to know, and the Flashcards are what let me do awesome in Bio - memorize the bio ones and it will make a big difference in your scores.
I'll be frank though, Kaplan doesn't do a very good job of PAT or RC prep. The Kaplan PAT tests are all waaaay easier than the actual DAT, I don't know how I got an 18 on the actual test, hehe. On the Kaplan tests I got Angle Ranking perfect the vast majority of the time, but when it came to the real test, I was just stunned at how much more difficult the questions were! It was like all 4 of the angles were the same angle - way more difficult than Kaplan. I'd suggest getting CrackPAT for this, I hear it's really good (though I haven't used it myself). For RC I didn't find the Kaplan method to be that great for me, and I actually did worse when I tried the Kaplan method, but RC is weird like that, everyone finds a method that they like and they run with it.
If Science and QR is what you're concerned about, and you feel like you might need a little help sticking to a schedule (I know I did), Kaplan is really worth the pretty penny for the class. Coincidentally, I actually just wrote a testimonial for them today, haha.
Though, many people have done amazing with other methods, so either way, it's still possible to do really well. Good luck!
