Yuu Shirota (1st Season Tezuka 2) and Kazuki Kato (1st Season Atobe 1) played a lot with this in the backstage videos.
Daisuke Watanabe (Tezuka) and Kyosuke Hamao (Eiji), both from the 1st Season Seigaku 4th cast, started to get shipped a lot after they both starred in 4 out of 5 movies of the Takumi-kun Live-Action movies.
Any former alumni who had co-starred in a Boys Love movie with Takumi Saito (1st Season Oshitari 1). So far, we have Yoshikazu Kotani (1st Season Kawamura 2, in "Boys Love"), Hiroki Aiba (1st Season Fuji 2, in "Sukitomo") and Ryunosuke Kawai (1st Season Hiyoshi A, in the adaptation of "Itsuka no Kimi e")
Fuji, played by Yuuki Kaida, and Yukimura are the only two third year guys to be voiced by a woman.
Dan is played by Yumiko Kobayashi.
Yukimura is played by Sachiko Nagai in japanese. In fact, he's the only captain or vice-captain voiced by a female.
Dawson Casting: Downplayed in the Film Adaptation. All the regulars are played by the Musical's Seigaku 2nd Cast, who were all 19-22 years old at the time, while the characters were supposed to be 14-16. Hongo Kanata as well was a bit older than Ryoma.
In case of the cameos in the film, the Musical actors who played random players were around their mid-20's.
Played straight in all the Musicals, but notably averted when Ruito Aoyagi (the 1st Gakuto) was casted. He just turned 16 during one of the plays.
Dyeing for Your Art: In the Musicals, Yuki Jutta (1st Jackal) shaved his head instead of using a bald cap.
There are also several musical actors that have gone on to become seiyuus, most notably Mamoru Miyano as Fudomine's Testu Ishida.
Not to mention that Momo's first actor in the Tenimyus, Eiji Moriyama, eventually became the voice actor of Fudomine's Shinji Ibu. Hello flashback! This gets exploited as a Celebrity Paradox in the Dream Live 1st.
For the Tenimyu 2nd Season, Eiji Moriyama returns, but this time playing Nanjiroh Echizen.
And from Shin Tenipuri, we have Irie Kanata, who is voiced by Hiroki Aiba (formerly Fuji in the musicals), Mamoru Miyano (1st Season Ishida) played also Ryoga Echizen, and Ryosuke Kato (both Kisarazu twins in the 1st Season) voiced Kousuke Tenjin.
For the Film Adaptation, many of the Musical actors make brief cameos during the film.
Keep Circulating the Tapes: Only the first 50 episodes were ever dubbed and released on now out of print DVD sets, these sets do not use the original Japanese opening theme, but intstead use the electric guitar theme from the dub, though the original Music is found on the extras on one of the discs.
The Other Darrin: In the Musicals. Aside from the actors graduating and Passing the Torch to the next cast, there were cases where the recast happened without a graduation.
"Remarkable 1st Match Fudomine" has some mixing in the cast: Since Yanagi Kotaroh was unable to perform due to his car accident, Kimeru stepped in as Ryoma, while Nagayama Takashi, who was originally playing Eiji, played Fuji. And then they called again Ichitaro Yamazaki to play Eiji.
"More Than Limit St. Rudolph Gakuen" had Kengo Ohkuchi as Tezuka (instead of Eiji Takigawa), and Eiki Kitamura as Kawamura (instead of Yoshitsugu Abe).
"Side Fudomine ~Special Match~" (2004-2005), was a remake of the original Fudomine play. This version has Endou Yuuya as Ryoma (he shared the role with Yanagi Kotaroh, who returned after his recovery). Also YOH replaced Sugawara Takuma as Tachibana, and Fujiwara Yuki played Kamio instead of Matsui Yasuyuki.
Starting with The Imperial Presence Hyotei, Date Koji was replaced by Seto Yusuke for the rest of the Musicals of the 1st Season, even when the original Hyotei Cast reunited for the special stage of The Imperial Presence Hyotei in Tokyo.
While there was a Rikkai B assembled for The Final Match Rikkai First, Masuda Toshiki replaced Yagami Ren as Yukimura, while Yamaoki Yuuki replaced Ono Kento as Yanagi, Acaba Mio as Marui and Toda Shingo as Jackal for the rest of the Season (they both played their parts for both casts). Averted for Kanesaki Kentaro, who was the only to play Sanada for both Rikkai casts during the entire season.
In Dream Live 7th, the Seigaku 1st cast made a special guest appearance, but Eiki Kitamura replaced Abe again because of scheduling conflict. It's even mentioned by the guys themselves on stage.