Seiyuus and their pseudonyms
There are many factors why seiyuus use pseudonyms...in fact the seiyuu name that you know might not be their real name at all!
Here in Japan, many talents have gei-mei - a name that is amusing and interesting to remember. For example, there was this one famous talent who recently passed away that every Japanese people knew whose name was Ikariya Chousuke. However, "Ikariya Chousuke" was his gei-mei (talent name) spelled いかりや長介, while his actual name was "Ikariya Chouichi," spelled 碇矢長一.
Seiyuus are not an exception. Some may keep their own name. Others, would change their name so it "sounds" interesting to remember.
Hmm, a bad example would be some guy who has a great song, but the record company finds his real name to be rather boring, like John Smith. So the record company gives him a kick-ass name like Elvis Bon Jovi. While his driver's license and birth certificate still says "John Smith," he is now known to the world as "Elvis Bon Jovi." Like I said, it's a bad example.
So, a seiyuu name that you might know, may not be his/her real name...which you won't know. For instance, Kawamura Maria (I guess Americans would best know her as Naga from "Slayers"). "Kawamura Maria" is her gei-mei, and her real name is Kawamura Shigeyo.
So, going back, yes it was obviously known that Uehara Tomomi = Mizuhashi Kaori. You can alterate Akane's voice within your brain (nounai-henkan) to Rosetta in "Kaleidostar" (to me, it was the other way around - Rosetta alterates to Akane). But then, there is no guarantee that even Mizuhashi Kaori is her real name...perhaps she maybe married and no one knows that, so her surname might be different now.
But for now, Mizuhashi Kaori is her "front name" that she uses in "normal" anime, while Uehara Tomomi (and countless others she has used such as Nanoda Sanae and Tsukishima Rio) was her "back name" that she uses in ero-related anime and games...
Some seiyuus choose to use their name for both worlds. For instance, Ueda Yuuji and Seki Tomokazu uses their name without change "Akane Maniax." On the other hand, others separate their usage for "front" and "back" like Hoshi Souichiro, who goes by the pseudonym "Aiba Tsuyoshi" as Shirogane Takeru in MuvLuv. There are numerous reasons that one can think of...to protect their image, because they were "advised" (meaning, "you'd better change your name or else we'll fire you) from their talent-production agency, or just for the fun of it.
Other notables include:
Tezuka Maki = Nabatame Hitomi
Misaki Rina = Itou Shizuka
which like in the "light," they tend to appear in the same ero-games as well.
If you know how to read Japanese, you can find the listing of seiyuus and their pseudonyms (should they have one, of course) here:
http://www.geocities.jp/gp44103/index.html