Bite the bullet and rewrite. Any guides out there will not be specific enough to your situation.
99 times out of a hundred if I realise I've made a massive design mistake and don't change it, I end up regretting it more than if I do. And since you'll take a copy/start from scratch again, you can always revert if you need to give up half way through!
Seriously, because this is a current app that is still (relatively) early in its development life and is all fresh in your mind it will very likely be quick to re-do. Also, sounds like this might be one of your first iPhone apps so you're likely learning new stuff every day - now you get to put all the new stuff in this app too. And finally, just imagine the work involved if you put another 200 hours into this as it is and then decided you had to rewrite!!
Hope that helps, and I bet you get it rewritten in less than a day!