The word straight from Microsoft:
It will be interesting to see if China tablet makers will push out Windows 10 for free or expect people to buy all new tablets to get it.
And what about those with dual-OS Android/Windows 8.1 tablets? How’s that going to work? Should people wait for dual-OS with Android 5.0/Windows 10?
Same-day update: In Comments, Vladimir Burnin pointed to this other tweet: