i dont see the kasha
of course Hashem can change the past or future. as far as His “Willingness” to change one but not the other…*We* do not live in all time at once. to *Us* the past has already occured and obviously from our perspective, cant be changed. the future, if there is such a thing, has not yet occurred and anything can happen, from our perspective it is not fixed, we have Bechira.
it makes sense to us to daven that someone should have a refuah. would it ever even enter our minds to daven that a certain person should never have become ill 2 weeks ago?