It is the same actress from the deleted scenes from ROTS, Genevieve O'Reilly. One of the rumor mill sites says "...the role has been re-cast with Genevieve O'Reilly, who looks dead-on like original actress Caroline Blakiston." She may have a substantial new role in Rogue One. We'll have to wait til December to know for sure!
Oh, and I made up the part about the Royal Guard not being Imperial. There's a lot of well referenced material from the expanded Star Wars universe regarding their backstory at
http://starwars.wikia.com/wiki/Emperor's_Royal_Guard/LegendsNot to be un-politically correct, but seems like Disney is going overboard with strong heroine characters and lacking strong male hero characters. What do you think is the reason? To expand the marketability of the movies and merchandise to provide appeal to both sexes? Is Katheen Kennedy driving this to "make things right" to make up for the lack of well developed heroine characters from past Star Wars movies? Don't get me wrong, I liked Rey's character and Daisy Ridley's performance, but looks like Rogue One will continue with the heroine as the focal character. As long as the story and characters are great, it doesn't matter to me, just noting the apparent bias.