If not, then the software is licensed under whatever license it was under when the operating system was released.
From an ethical point of view, it's worth thinking about whether you'd like to respect the wishes of the author(s) of the software, even if you are not legally required to.
IANAL.
Do the laws allow for retroactive changes?
In most places, probably not.
A license agreement is agreed to by both parties. If the terms of the agreement change, neither party is required to "re-agree".