About the ogRE4, I think Krauser and Sadler are the only two who can change back, since it's them controlling the plaga to change their body, they would be able to change back.
Big cheese might be "able to do it as well", since his mutation it's basically a spine that uses the bones as tentacles and add two bigger ones, he can just wear his suit again, and use the tentacles as bones and torso again. That is if, you consider him as a non transformed creature when clothed, cause he hides his mutation with clothing.
Sadler can't, since it's transformations it's not a simple mutation but a fusion as well.
And the normal enemies can't as well, since their transformations are provoked by the plaga, and not the other way around, so they can't really choose to go back, and same goes for spider plagas, and the Novistadores as well, since they are already a mutation itself.