There are huge changes, as there are from Valis III PCE to MD, but the SFC version is still based on and built from assets of the PCE version, just like Valis III MD was. It may not be as faithful as the MD version of Valis III was, but it follows the same downgrading design. They just gave this game all the less space for what it needed compared to the MD port of Vallis III.Valis IV may be a port, sort of, but it has some MAJOR changes that you fail to mention. Most obviously, there are three playable characters you can switch between in PCECD Valis IV, but the SNES game has only one. The SNES version has an entirely different and more complex powerup system instead, though, while the PCECD game has a quite basic one (the main depth are the three characters). The SNES game also removes almost the entire story -- there's an intro and an ending and that's it, all other cutscenes are gone. And as for the level designs, they are similar, but different. The stage themes are the same, sure, but while similar, the level designs aren't exactly the same and because of the other gameplay differences they play differently.
I'd say those are some huge changes, myself!
The stage layout and gameplay is different, but it is still the MD's version of Valis II PCE, with the same stage themes/art, bosses and character designs.True, but my point was that the levels are far from identical to the PCE game. It's not just the same exact game just with different graphics, the levels and gameplay are somewhat different as well.
The PCE Valis II came out before Syd of Valis/Valis SD, but Valis began as a computer game series and the PCE version is a re-imagining of the second game and from which Valis SD was based off of. The PCE Valis I is not based on the MD/Genesis Valis I at all. Valis I MD was made after Valis III MD and rehashes the engine and player sprite. The PCE Valis I, like the MD Valis I, is a re-imagining of previous versions of Valis I (including a Famicom version), which first came out in 1986(?), but uses a brand new engine and completely new graphics and gameplay.It's worth mentioning that while Valis II through IV were on Turbo/PCE CD first, Valis I's PCECD version was released later, after II through IV, and is an enhanced port of the Genesis title. It's also the only Super CD Valis game, and for some bizarre reason is the only Turbo/PCE CD Valis game that does not support saving, so you have to play it through in one sitting (annoying!). Of course, none of the SNES/Genesis versions of II to IV have saving, so it's a major advantage of the TGCD versions.