As far as I know, and I could be wrong...
yes1) Will I automatically get upgraded to KEAOS v4.0+?
no2) Will I be able to play any blu-ray content imported via the M500 on any of my other 1080p players?
yes3) Will I need to upgrade my other 1080p players with M500 or M300 players in order to play any imported BD content?
not yet, ultimately yes4) I have a ton of HD home video. Can I import that content? If not, is that a permanent shortcoming or something to be added soon?
yes, but it's trade-in value on an M300 is less than what you'd get for it on an M500.5) Can a 1080p player be upgraded to an M300 at a discounted price or does that only apply to M500's?