The corrals go by what time you put to finish in on your registration. So, if you picked the latest time, then you will automatically be placed in the last corral.
And to get into the first corral, you need proof of finishing time from a previous race.
I believe last year,i picked my time to finish as the next to the last choice -- so i purposely wouldn't be in the last corral. I was in "corral B" last time, which i loved, and that's what i am planning for next March as well. However, I know Lauren and her DH were in the last corral, and they did just fine.
I think there were about 900+ people who finished after me, which i was happy with..My time was about 3:15. I am shooting for 3:00 this year..
You can not switch corrals once you are assigned, they have people who will be checking your bib number - as your corral is on your bib.
On a side note - just wanted to share -- i joined Weight Watchers in March after i got back from the Princess, and just hit my 10% goal (which is 19 lb.s) - actually , i passed it - i am down 20lbs !!! I am so excited and feel so much better -- now i am shooting for the next 10lb mark - I hope to be 30-35lbs less for the 2010 race as i was for this years.