The bounty reasons are the same throughout the network. This bounty reason is easy to understand on sites like Stack Overflow: Releases of new versions of a software package can make answers referring to an earlier release outdated, that's not an uncommon occurrence.
An analogous thing can happen in mathematics, a conjecture can be proven/disproven, sometimes even something that was accepted as a theorem turns out to be false. Of course these events are very rare in comparison to new software releases. In such cases, the "out-of-date" bounty reason is appropriate.
Changing the question after an answer was posted is - with some exceptions regarding clarification, or addition of constraints that were forgotten in the original post - strongly discouraged. Everywhere on the network.