I remember them involving secondary teams in trilogy development. Thats pretty much was reason why Versus was pushed/remade into FF XV
They borrowed manpower from versusXIII/XV at the time of XIII's initial development because they had to rework a lot of stuff when they created the Crystal Tools and shifted the title off of the Unreal Engine. At the time, versusXIII was still in its conceptual and pre-pre-pre-alpha design stages - essentially, not a whole lot was actually happening with the game.
Despite versusXIII having been announced alongside XIII, the game that would become XV wasn't even in development until after XIII-2 was completed. Contrary to popular belief, versusXIII/XV hasn't been in development since 2006. It's been in development since late 2011. The fact that we're seeing XV in maybe a year, gives the game a four year development cycle, which is right in line with a completely new, built-from-the-ground-up game that switched both platforms and a proprietary engine.
However, your main point is still off, because they wouldn't be pulling a dev team in full swing for a collection. The KH ReMixes have a brand new team, and they're working with existing content, so the teams are pretty damn small. Even less work would need to be done for a XIII trilogy to be updated for the next-gen consoles, really.