Related: When is it allowable to change the rules?
As noted in the linked meta post above, it's typically particularly frowned upon to significantly change the rules of a challenge after posting it. There's no hard and fast rule on this yet, so the purpose of this meta post is to enact at least some kind of policy to determine when drastic edits should be rolled back.
Here, we define "rule change" as not any type of edit but any edit that causes one or more reasonable hypothetical answers to become invalid beyond repair. Note that the "beyond repair" clause means that this does not include minor rule changes such as input/output format. Note that the wording of this definition also (intentionally) does not include edits that give answers additional liberties.
There are a few "poll-style" answers here to vote / comment on:
Should rule changes that don't invalidate existing answers be allowed?
Should rule changes that invalidate one or more existing answers be allowed?
Adding your own answer is encouraged, though, possibly addressing some of the following questions:
Exactly what it says in the title: If you do think that some rule changes should be acceptable, which ones specifically and in what situations?
If we're going with "rule changes are only allowed if they're minor enough," how do we define a "major" rule change?
If you think the policy should be something other than something that was mentioned here, what is it, and who gets to decide which rule changes are allowed?