To my knowledge, you can't get a rider swap in a single rider line. But you can combine FP and rider swap.
In a related topic, I am wondering the best fastpass strategy for California Adventure. I have a 5 yr old son and a 7 yr old daughter. My son won't do Guardian of the Galaxy, but the big sister will try it.
Staying off-site. Planning to go on a day when California Adventure does not have EMH and planning to be there at rope drop.
My plan is to go to the GotG first, get a child swap and let my wife/daughter do the ride as stand-by. While they are on GotG, go get FP for RSR.
Depending on the return time window of RSR, the stand-by wait at GotG and if my daughter liked the GotG, maybe ride it again stand-by and get another child swap (we'll have two child swap for later in the day). If line is already long, then save the one child swap for later and do something else before RSR.
Please share thoughts. Thanks!