Although it seems that every PV in Project Diva games is just background for the song, not only they’re painstakingly built, most of them have something unique about them; new subject matters, new situations, new pairings, etc. One of the novelties is that some PVs are actually continuation of another PV.
Here are the ones that I’m aware of:
From Y To Y (PD2nd) -> Starduster (PD Extend)
This one is the most obvious. In From Y To Y, Miku sees her astronaut boyfriend off. In Starduster, well things happen. Miku presumably dies of illness (the specific module for this song has one of her eyes of different color), and the astronaut couldn’t make it in space but managed to send his ring back. They’re essentially reunited in death, both their rings return and they share the same graveyard (Are you crying now? ^^;)
Both songs are done JimmyThumbP and they’re indeed meant to be a continuation of each other (so the PVs just respect the original concept).
Hello Planet (PD Extend) -> Godly song (PDF)
The Project Diva PV for Hello Planet is like a 3D remake of the original PV (albeit with many scenes cut).
Original PV of Hello Planet:
Project Diva PV of Hello Planet:
Godly Song PV:
The reason I’m showing the original PV is that Godly Song is more obvious as a continuation when you look at the original PV instead (although you can still catch some of the hints in Project Diva PV). Notice the earth shot, the props (the sleeping chamber, the pot of plant, some of the items the chibi characters use), the weather conditions (rain, strong winds, etc), and finally the tears to beanstalk thing.
Unlike From Y To Y and Starduster, the two songs have different creators, so it’s Sega’s own idea to make the continuation. Like From Y To Y and Starduster though, they’re separated by installments (2nd -> Extend, Extend -> F)
Voice (original) -> Voice Diva Mix (PD 2nd)
Project Diva PV:
This one is slightly different from the previous two entries in that the Project Diva PV is actually a continuation to the original PV. In the original PV, Miku is still a child and she is sent away to keep her safe from war while the robot protects her home. In Project Diva PV, she grew up and returned to her abandoned home, her robot nowhere to be seen.
The Secret Garden (PD 2nd) -> Sakura No Ame (PD F2nd)
This one is debatable, especially to naysayers.
Both The Secret Garden and Sakura No Ame are songs from the first Project Diva. What’s interesting between these two songs is that at first there was no explicit proof of continuation between them. The only thing that they had in common was the school setting.
Here’s how the continuation was created: For some reason, Sakura no Ame disappeared from the first Dreamy Theater, the PS3 HD add-on to the first Project Diva (probably licensing issues). Since then, this song has disappeared from Project Diva series until it finally returns in Project Diva F 2nd.
In Project Diva F 2nd, the PV for Sakura No Ame is recreated instead of just being remade to HD. Firstly, the default costume in this PV is no longer the default costume in the first Project Diva, but it becomes one of the default costumes of The Secret Garden in the Project Diva 2nd. Secondly, the PV is updated to be about Miku graduating, making it look like a continuation of The Secret Garden where she’s still a student.
One of The Secret Garden’s default costumes in Project Diva 2nd:
Project Diva F 2nd Sakura No Ame PV:
Note that in the first Project Diva, this costume is not the default costume of The Secret Garden, but Ichibyou No Slow Motion.
The default costume for The Secret Garden in the first Project Diva:
But these two costumes become the unlockables, hence the default costumes, of The Secret Garden in Project Diva 2nd (since Ichibyou No Slow Motion is not included in 2nd).
Ichibyou No Slow Motion btw, is about Miku going to school in the morning, so yet another way is to think all these three songs as continuations. In 2nd, The Secret Garden is ‘retrofitted’ to be a continuation of Ichibyou No Slow Motion (with Miku wearing the same school outfit as Ichibyou No Slow Motion). Then in F2nd, Sakura No Ame is retrofitted to be a continuation of The Secret Garden (again by having Miku wear the same outfit).
And that’s it. There may be more, but these are the ones that I’m aware of.