It might be too late now, but I've successfully negotiated (before signing) retention deals like this to be pro-rated in the event of non-voluntary termination. It's perfectly reasonable for exactly this reason, and companies have no legitimate reason to deny it.
Agreed, this is/should always be the deal basically - "here's a bonus so you stay while we transition" is the only reason many folks stay post aquisition. It should not be revokable, especially last minute.
