Folks do opt not to have their AAP-eligible kids move to an AAP center or even have AAP services in their base school. But you can keep your child technically on the AAP list, so to speak, and opt INTO AAP services or a center at any point up through middle school. So -- please don't confuse opting out of the pool with opting out of sayiing yes to AAP services/ centers. Your child, once eligible, remains eligible -- you do not HAVE to take up the offer of AAP services. But if you want to leave the door open to your child moving into AAP services/center later on, don't formally say "I want my child taken off the eligibily list."
Talk to the AAP office at FCPS about how this works. Your child can keep the eligibility and opt INTO the services at any point, I believe, but if you totally remove your child from the list of eligible kids, you might end up with problems getting your child into AAP later on if that becomes the right place for him to be.
For instance, I know of kids whose parents did not send them to an AAP center until fifth or sixth grade because the parents felt the kids weren't ready until then. These kids were able to move right into centers because the kids were still AAP eligible.