That sounds similar to my other favorite work/legal story, the crux of which is that initial employment counts as consideration (in most jurisdictions) and thus can be the basis of a contract, whereas continued employment does not and can not. This is directly related to why many companies technically fire employees during an acquisition and then re-hire them as part of the new parent.

IANAL and this is not legal advice. I just had to live through a situation where these distinctions were highly relevant and important.