The answer is yes, the casinos most certainly can change the payout schedules on slot machines at their convenience or, as you say, on demand. But there is some effort involved. With server-based slots (SBS), slot managers could change a slot machine's hold percentage remotely via software with some keystrokes.
The truth is, yes casinos can control a slot machine or rig it only to give players small wins. Some casinos work with developers to create exclusive games. This gives them even more access to a game's code and RNG.