Originally Posted by Kedreeva
Yes she can't morph into a rooster. She either is a hen, has always been a hen, and will always be a hen, OR she is a rooster, was always a rooster, and will always be a rooster. If you have young birds, it's possible you sexed the hen wrongly and do have a rooster. However, if you are sure it is a hen, rest assured that there will be no gender change.
Sometimes in groups of hens, if there is no rooster, one of the hens will 'take charge' so to speak. She will become the dominant hen and pretend some of the roles of roosters, including awkward crowing attempts and 'mating' with the other hens.
Actually, as an odd biological sidenote, hens have two ovaries, but only use the left one for producing eggs. If this ovary ceases to function due to illness or disease, the right ovary will become active. For some reason, this second ovary can produce large amounts of male hormones and cause the hen to develop male plumage and other secondary sex traits. The affected chicken will even begin to crow like a rooster. Having said this, it is true that the hen will never become a "true" roo, it will lack the basic parts needed for reproduction (although some will argue that the hen will develop those as well).
Interesting as this is, it is exceedingly rare, something like a 1 in 10,000 chance. I will agree with others here, barring a mistake in sexing, your hen cannot become a roo and the behavior you are seeing is the establishment and maintenance of the "pecking order" amongst the hens.