Without the “what if” part, “changed” is past tense, “has changed” is present perfect, and “had changed” is past perfect. Those would be easy enough to describe. But adding the hypothetical “what if” makes the time frames of those sentences much more similar.
“What if it changed?” could actually refer to something that had already changed in the past or something that could change in the future. The response still has to be about what the responder will or might do in the future.
“What if is has changed?” Has to refer to a change that has happened by the time the question is asked. The response to this also has to refer to future actions that the responder will or might do in the future.
“What if it had changed?” Refers to a change in the past and what actions would someone have taken after that change but those actions were in the past too. The response to that question would typically be something like “Well then I would have ...”.