For National XC you do need to pre-enter Sport; it is only open and fun that are available on the day.
You will need to finish in the top xx of the sport cat to progess to expert and so on to elite. With the Masters cat having been abolished at National races that just got harder! The BC website rankings page details the exact information.
The cat you can enter can be dependant upon your age, but guessing you're younger than 39, sport it is. 40+ and you could go straight into Vets.
In terms of National Champs, the sport race is separate and pre-entry as I recall. However you would have the option to race senior which is pre-entry about 3-4wks before the race; you would effectively then be racing with the elites/experts.
At regional level you can usually race in a different category if you so desire, e.g. expert instead of sport but you wouldn't get any ranking points and therefore this wouldn't help you with your overall BC ranking position at the end of the year.