What happens if one of the targets of Role Reversal is enchanted by a spell that changes its controller?
Role Reversal will actually exchange the creatures.

All control changing effects, including Role Reversal, are the same sort of effects: continuous effects. Control Magic has a static ability that creates a continuous effect as long as it is on the battlefield. Role Reversal creates a continuous effect at resolution that, as the quoted ruling says, lasts indefinitely.

So, determining what happens with both of them is determined by the Interaction of continuous effects rules. Both of these are control changing effects, so rule 613.6 applies:


  
  613.6 Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp.
  
  
  613.6a A continuous effect generated by a static ability has the same timestamp as the object the static ability is on, or the timestamp of the effect that created the ability, whichever is later.
  613.6b A continuous effect generated by the resolution of a spell or ability receives a timestamp at the time it's created.
  613.6c An object receives a timestamp at the time it enters a zone.
  
  


The end result is fairly intuitive: the Role Reversal was cast later, so its effect gets applied later and it determines the final outcome.