Well, first off, there's no need to reformat your hard drive just because you switch from MPC to OGG. Both formats will coexist just fine in MJ. I have files in all formats (MP3, MPC, OGG, AAC, APE, FLAC) in my library and have no problems with any of them. Just start encoding new files with OGG. For the CDs you've already ripped to MPC, maybe re-rip/re-encode a few to OGG every week until you've replaced all the MPCs.
As for MPC becoming useless, I doubt that will happen...anytime soon anyway, but it will probably never become as widespread as MP3 or OGG. You are right, though, there are not that many programs that support MPC, but there is a small but dedicated MPC contingent out there that will always makes sure you'll be able to play them in MJ (or Winamp).
What quality setting were you thinking of encoding your OGGs at? Be aware that almost all of the sound quality tuning has been concentrated on the lower bitrates (-q 4 and below). This is not to say that higher bitrate OGG (-q 5, 6 , 7) doesn't sound good, but from what I've read over at Hydrogen Audio, there is a lot of tuning that can be done for those higher quality settings.
Ultimately, Matt's lossless suggestion is the way to go if you've got the hard drive space. Then you'll never have to worry if you chose the right format.
Rob