Live conversion of frame rate (conform style) or frame rate based playback speed adjustment for video content

[Drunken Hamster]

Basically, I'm incredibly nitpicky about things and I've decided I don't like non-integer frame rates. I also generally don't like low framerate content, either, but you can TELL when something like a movie is sped up as opposed to a different frame rate. So... What I'm really asking for is either A. A way to play a movie back at a specific framerate (allowing it to adjust playback speed) or B. A way to quickly and simply convert movie files up to 4gb from the standard 23.976 to 25fps.

I don't wanna faff with settings and trying to guess what it was before the conversion so I'm not reducing the quality by converting, nor do I want to delve heavily into whether or not I should change the file type from MP4 to MKV or change the codecs or anything, I literally just want to be able to go into "playback speed" and type "25 fps" and press "okay" and it starts working. Or be able to convert a movie without degradation of quality in roughly the same time it takes to download a new one. (which is 5-15 minutes)

So... Any advice?