power has nothing to do with it. like how boltboutamon comes from an ultimate+mega, same for milleniummon.
if we ever see rasenmon himself become fury mode, then we have instances of both rasenmon -> fury mode and fury mode -> rasenmon
right now we only have fury mode -> rasenmon through its profile, so its only one way. doesnt mean it wouldn't change in the future though.
the evolution, in the profile, is a permanent evolution of fury mode to rasenmon
and example that isnt permanent is omnimon zwart where it says its an omnimon temporarily boosted with the black digitron
also an example i just thought of, apocalymon -> mephistomon. thats a mega digivolving into an ultiamte, so gets weaker.