Not sure what the problem is here, Hex. If you're narrating in present, ie what's happening today, and you want to say it's as a result of something that happened yesterday, you have to go into past tense to do it, (ie "Yesterday we sold the horse, that's why I'm walking") unless you're striving for a "Well, I says to him" or a Damon Runyon effect.
So if the chapters are dealing with things which happened before the story starts, they ought to be written in past tense. As long as you make it clear that these chapters are the past, I don't see why it should cause any difficulty at all.
Gah: HB beat me to it.