It's interesting, because MegaTen games are always like that. There's two games that are almost identical, and then something completely different. For example, Megami Tensei I and II, and then Shin Megami Tensei I and II happened, and then III and IV moved to polygons and third person exploration.
Then there's Devil Summoner. The original and Soul Hackers, and then the Raidou duo happened on PS2. And Personas 1 and 2 were similar, and 3 and 4 were something different.
It's why I'm so excited for Devil Survivor 3. 1 and 2 were really similar so I imagine the third game will be as drastic a change as Persona 2 --> Persona 3.
While we're on the subject of changes to Devil Survivor, I think it'd be interesting if it adopted a combat style like that of the Final Fantasy Tactics series.