Metal Gear creator Hideo Kojima explained a bit more about the upcoming Metal Gear Rising: Revengeance via Twitter, including its transformation and mechanics.
Metal Gear Rising originally started as Metal Gear Solid: Rising and was in development by Kojima Productions. A few trailers and demos of the game were shown years ago, running at 30FPS with the typical environments of a Metal Gear game. When the game shifted hands to Bayonetta developer Platinum Games, Kojima told them to make it run at 60FPS. It also features a new atmosphere and color palette.
Platinum Games also made quite a few changes to the game. Metal Gear Solid: Rising was originally a prequel to Metal Gear Solid 4, following Raiden's transformation into a cyborg ninja. The new story follows Raiden post-Metal Gear Solid 4 as he's sent to a developing country in the midst of a civil war. However, he unknowingly hast to face an army of cyborg ninjas like himself. The original title also featured a mix of stealth and action. When the game came over to Platinum Games, it was changed to be a Metal Gear Solid 4 sequel about Raiden and the stealth was dropped completely for a total action.
One key mechanic in the original version of Metal Gear Rising was a cutting mechanic that allowed protagonist/cyborg ninja Raiden to cut almost anything, from the environments to enemies to even fruit. While the newer version is a lot faster paced and it could easily be assumed that this mechanic was cut out (pun intended), Kojima has assured that you'll still be able to cut anything. This new version will let you cut up enemies and objects anywhere in real time.
As per the name change, Kojima explained that the new title was for a new protagonist. While Metal Gear Solid (exception being Metal Gear Solid 2) focused either around Solid Snake or his genetically identical predecessor Big Boss (aka Naked Snake), Metal Gear Rising focuses on Metal Gear Solid 2 protagonist Raiden. The new title hopes to distance the game from a typically Snake-dominated series and avoid it being put down as a "Metal Gear Solid spin off".