If I'm simply changing dates on a ticket, I pay my change fee and expect it to be re-issued in the same fare class. Even if prices have changed in the meantime it should make no difference. If my fare class is unavailable, I'll expect to pay a fare difference to move to a higher fare class.

I have never heard of any other airline rebooking in a lower fare class. I can't see any reason why it would be in the interests of the customer.

Of course, if I choose to change my routing, then all bets are off and a full reprice is in order.