Topic came up at a local recently. Basically someone was talking about how the first iteration of a Capcom title always sucks. They mentioned Alpha series as an example. However, to me the Alpha 1, 2 and 3 were stand-alone titles in the Alpha series. Now it's probably not hard to make a case for A2 being an upgrade of A1, but to me, Alpha 3 is much more than an upgrade. In other words, Alpha 1 cannot be seen as "Vanilla Alpha" in comparison to Alpha 3, at least in my opinion. But this raises an interesting question: At which point does an upgrade become a sequel?
WW -> CE -> HF are clearly upgrades. But is SSF2:NC an ugprade? It's on a new hardware platform. It pretty much has a new engine. However, it's still a Street Fighter II title. So going by the naming, it should be seen as an upgrade, no? Or does the name simply not matter? If it's a case of aesthetics, what about SFIII:NG to 3S? Is CvS2 an upgrade of CvS1, or is it a sequel? How about the EX series?
What about games like Tekken and VF where the core mechanics hardly change at all across iterations? What about KOF 94 to 98? Vampire Hunter -> Vampire Saviour?
I can't actually think of stable criteria to distinguish sequels from upgrades. To me it seems like a case of marketing + what the majority of the community thinks, regardless of whether or not stable criteria exist.