Ionesco describes the actual physical change that the characters undergo.  The skin morphing, the color disfiguration, as well as the mental change that becomes part of the evolution are all a part of this process.  I think that the more interesting element is the notion of the conformist view that overtakes the social order.  Berenger is forced to see his friends, his co workers, as well as the woman he loves all become something he isn't and will never be.  This is probably one of the most powerful changes that happens in that everyone in the play is becoming something else and that Berenger is staying the same, forced to witness the change.  This change is something that replicates itself on a physical level, in that everyone in the social setting is becoming a rhinoceros.  In the gradual change where Berenger is the only human, this change becomes more pronounced and profound.  On an emotional level, Berenger endures change in that his desires at the start of the play are quite conformist.  He wants to hold down a good job, get married to the girl of his dreams, and live a very conventional, perhaps even mundane life.  Yet, these dreams begin to fade as there is constant change around him.  Berenger undergoes emotional change in this in that he vacillates in wanting to be what everyone is, even seeking to try to alter his own physical appearance.  Then, when understanding that "he is what he is," Berenger becomes resilient in being the last human, evolving into a type of "superman" from being a very ordinary person.