They've already replaced Dalvik with ART for bytecode and it was mostly transparent from what I understood.

But popping up with a completely new platform API without any backwards compatibility would be very bold and risky. There would have to be a good compatibility story there.

Also there is talk in the article about a POSIX layer. Now that's easier said than done. But it is Google we are talking about so they certainly have the manpower and resources to try.