You can swap in alternative encounter sets to replace their regular version. When doing so, you must replace all cards in the set. 

When playing a campaign: When starting a new campaign, swap out 5 random encounter sets. Make this change in the Choose Difficulty step (before selecting your character). It applies for the entire campaign. 

When playing a stand alone episode: Swap out 2 random encounter sets. 

The alternative encounter sets use the same set icons as the regular sets, but are colored in red. Each set overall is no harder than the regular sets, but will challenge you in new ways.