I think you fall under F2B category in immigration status if you file it today. which has a long waiting period (depending on the volume of petitioners in the region). If you are in the Us and maintaining your status, you can file it to change your status. but you have to maintain your status till it changes. Good thing doing so is your application will be in the line as green card holder parents sponsoring an un married child above 21 and the countdown starts from the day you filed the petition. by the time when your parents becomes US citizen they can reopen the same file but the process will be little shorter since you have been waiting already. This is what i know. please do not quote on me. Jai Hos!