Now personally I haven't seen the play myself, however there is someone who has they have kindly talked about what happened during the play and what changes were made to the story:
Quote below taken from this link: http://crocus29.tumblr.com/post/101733129216/update-dr-stage-play
30/10 I went with sis & fy-chan to watch the stage play at Nihon Seinenkan. We bought the tickets a bit late and they were almost sold out, but I was glad we could get 3 seats in a row and sit together ^^
When we got there, as expected, it was super crowded OAO
[Fy and I met in real for the first time here and she got me gifts and even drew me something QAQ! Meanwhile, I completely forgot about getting gifts and I still owe her a request… Omg Fy, thank you so much! I’ll definitely pay you back later! >A<„]
Either way, for the play, they stuck pretty close to the original storyline, but added and changed a few things.
One major change was the addition of a new rule:
'Whoever voted for the wrong person during the school trial will be executed.'
All the trials stayed the same as in the game/anime, but this new rule resulted in Ishimaru being executed along with Mondo in Mondo’s trial because he didn’t want to vote for Mondo (them being brothers).
This in turn resulted in Ishida not appearing and Celes’ case not happening.
Celes and Yamada are executed using the same rule during Sakura’s trial [They added new content in which Celes pretended that she killed Sakura to mislead everyone into voting for the wrong person.]
Personally, I think this new rule’s quite a good move to make the story shorter, considering the stage play itself already ran for almost 3 hours =u=
Thennn… Instead of giving everyone a special execution, the murderers are all executed using spears. The same way Mukuro is killed near the start of the story when she tried going against Monokuma.
One other major change is that in the ending, Naegi didn’t allow Junko to kill herself. So Junko lives. What happened to her afterwards is not told, though. The story ended when Naegi and the survivors (same as in the game/anime) opened the school’s doors
Well, this is the first time I’ve ever seen a professional stage play live and I think it was awesome! XDD Certainly a different feel from when you’re watching it on TV oAo!
The costumes and sets are done well and the movements/scene changes are executed beautifully. The actors are really good! ^^
The Monokuma dancers are a fun addition, too >:-D
[Uoo, made me wanna watch Butai Basara 4 which is being held at around this time too, but I didn’t get the tickets for that… >A<]
Well, all in all, it was a fun experience ^^
Ok, anyway, I’ll just write some of the Ishimaru & Mondo parts here (These parts are in the play but not in the game/anime). Looks like usable materials for drawing, so I’ll just note them down before I forget :p
1) Entering the Sauna scene, Mondo & Ishi bickering with each other:
Mondo: Your pants are too high!
Ishimaru: Your pants are too low!
2) When Mondo admitted that he was Chihiro’s murderer and accepted his fate, Ishi grabbed him by the collar and tried to stop him.
Mondo: (ready to die) I’ll need to tell aniki… That I’ve made a new brother.
3) Like in the game/anime, Ishi refuses to accept that Mondo is the murderer and didn’t vote for him, even though Mondo told him to.
So with the new rule, he’s executed along with Mondo (they died together). Their last words to each other:
Mondo: (surprised) Kyoudai… Why—?
Ishimaru: Aren’t we… brothers…?
So the most important changes in the play are:
1. Because of the new rule added in, Ishimaru was executed the same time as Oowada.
2. Because Ishimaru was executed, the entire Case of Chapter 3 is skipped out in the play.
3. This means that Celes and Yamada are involved in Chapter 4's case and are both executed for picking the wrong culprit.
4. Junko never executed herself and lives as Naegi refuses to let her die.
I suppose this was necessary to shorten the length of the story and since Case 3 was the weakest one for me personally as well as being the most filler of all chapters then its not so bad if they miss out this chapter.
However for me the biggest surprise was letting Junko live because for me this pretty much confirms that Kodaka (writer of DR) regrets killing off Junko (He wrote the script for the stage play as well.) at the end. Probably so that he could've continued using Junko as the big bad of DR. As I don't believe he ever thought DR would become as popular as it has.