This is completely wrong on many airlines. I rebook myself in response to IROPS all the time, often via the mobile app but sometimes using a mobile browser. If I have to speak a human to do a simple rebook of same origin/destination, that tells me I'm dealing with a primitive airline.

And no, at that point (following a cancellation), you do not need your same fare bucket available (what the OP is getting at). You need seats available in your cabin of service but do not need to upfare after a cancellation.

Of course if you're trying to change something else - flying to a nearby airport that isn't really a coterminal, trying to refund the entire flight, trying to change to a lesser/greater cabin of service, trying to select a flight that is farther away from your original flight than other available ones, etc. - you might need a human...