I thought I had read somewhere that the use of rider swap to "double your FP+" in the way
@Andrew M described was something Disney had recently cracked down on, and that the riders in a party who wished to swap/switch from the FP+ line now had to
all have FP+ for that attraction. Can anybody confirm whether that's the case?
UPDATE: I found an article (dated June 2018) stating that Disney has changed rider swap/switch to a digital form, and is officially requiring that all riders using the FP+ line and intending to swap/switch at that attraction must have FP+ for the attraction already. Unfortunately, the article is on a site that is blocked by this forum, so I can't link to it. (You can find it by Googling the article's title, " Walt Disney World Switching To Digital Rider Switch Pass or Baby Swap At Attractions To Combat Fraud.") At least one of those who commented on the article confirmed that the rule is now being enforced (although in the interest of full disclosure, other commenters claimed that such enforcement is spotty): they tried to enter the 7DMT FP+ line to do rider swap with just some (not all) of the intended riders having FP+ for it. The CM insisted on scanning everyone's MagicBands to confirm that all of the intended riders had FP+ for the attraction, and those that didn't were denied entry to the FP+ queue (and would have been offered a rider swap window that wouldn't open until after the current standby wait time had elapsed).
Obviously, YMMV.