In my experience, it doesn't take cosplayers long at all to adopt a new character. At last October's New York Comic-Con, I saw several Korras and an amazingly good recreation of an Engineer from Prometheus
. (At least, I think it was a good recreation; I haven't seen the film, but the cosplayer had a really elaborate makeup job.)
It would be easier for cosplayers to do Castithans, I'd think, since that's just makeup, wig, and contact lenses. The prosthetics for an Irathient look would be a bit more involved.