In Novice A, you have 60 days after obtaining your CD to continue to show in Novice A, during which time you can show as many times as you would like in A, obtain as many HIT's as you can possibly obtain. After those 60 days, however, you MUST either 1) move up to the B class, or 2) move up to Open.
If you show in the B class, after obtaining your CD, you can continue to show in B UNTIL you get your first HIT that occurs OUTSIDE of the 60 days after obtaining your title ( again - a 60 day grace period to obtain as many HITs as you can without being forced to move up) - after that, you HAVE to move up to Open once you obtain a HIT. The only caveat being I believe the AKC will allow you to continue to show in ANY class that you have already entered PRIOR to obtaining that HIT (ie if you entered 4 shows, but obtain a HIT in the second, you can finish out the other 2 shows in that class if they all occur in a row, I believe). An A class competitor moving up to the B classes is under these same rules.
Of course, the minute you obtain your first open leg, you can never show that dog in Novice again. In addition, even if you are within your 60 days that you can still show in Novice A, if you trial your dog in open (even if you do not qualify), I do not believe you can go back into A, you have to the NovB class.