I've seen POs swap projects, new projects created, weeks of planned work cut, people added to workforce, people cut from workforce, and much, much more in less that a fortnight. Is that bad? it is, if you're lost with no hints. On the other hand, it is also an opportunity.
If you spent some time in serious planning you have a good basis on which you can build your ability to steer the change. That does not mean that you need a good plan (which could now be completely worthless), but that you have a solid knowledge of what's on the table, and that means more information, which translates into a conscious decision instead of a jump in the void (or at least it mitigates the risks).
And let me say that again:
The waterfall model is wrong!