If you are getting into college football officiating and still want to work high school, just a new rule of thumb and/or attitude you must take: you are now a college official who happens to do high school, not a high school official who happens to do college - I don't care how many "big" games you get in high school.
With that being said, if you're going to work both NFHS and NCAA, I've found that by dedicating 80%-90% of your study time to NCAA, you can easily drift back to the NFHS rule, since the NFHS rule is usually a simplified version. (Examples: intentional grounding. NCAA has exceptions, NFHS does not; NCAA has crazy penalty enforcement on tries and overtime after a change of possession; this does not exist in NFHS. See what I mean?) Remember the NCAA rule because the NFHS rule is usually less complicated. It's easier to "take off" complexity rather than "add on," if that makes any sense to you.
Keep working hard at it and it will come. Trust me, you will do fine as long as you put the effort into it. It generally takes 2 yrs to fully MASTER the NCAA rule book. I equate learning football rules to learning math problems - you only get better by doing practice problems. There are a good amount of pre-season tests floating around on the internet. Get a hold of them and start taking them!
Also a word of advice: if you apply a Friday rule on Saturday, may God help you. haha
Good luck and welcome to college football my friend!