This is factually wrong. You can grandfather it in.
What part of anything they have said to date, indicates this is happening?
Further, considering 1.2 (or 2.0 or whatever we want to call any and every version of the leaked language) is not remotely 'Open', the point is moot.
1.0, must remain untouched, or strengthened, everything else is an unnecessary concession.
No, the OGL 1.2 does not deauthorize OGL 1.0(a), that is a separate move by WotC. Nothing in OGL 1.2 invalidates 1.0(a).
However, even if we accept the OGL 1.0(a) is deauthorized (which I don't) it only stops anything new using that license. Just move over to either the CC or OGL 1.2* and your all good.
*Again this assumes the OGL 1.2 is improved as needed.
Everything under the 1.0, would remain there, in some legal limbo, if the 1.0 is deauthorized. Going to CC (and no I wont go to 1.2) wont fix that. Nor will ORC.