Stage Morph
Stage Morph is a feature in Super Smash Bros. Ultimate, available as a toggle in the Rules selection, which was revealed during the Smash Bros. Direct on August 8, 2018. It allows players to choose two different stages on the stage selection screen for a match, and have these stages transform into each other mid-battle.
The frequency of the transformation can be set to Off, which disables the feature; Random, which transforms the stage at random intervals; or Every [interval], which transforms the stage at a configurable interval (with a default of 1:00).
The transition itself bears some resemblance to the transition of Castle Siege or Reset Bomb Forest, though instead of a walk-off, it simply drops out the current stage's platform as the next one's appears.