No, slot machines are not rigged. They use Random Number Generator (RNG) technology to ensure each spin is entirely random and independent of previous outcomes. The casino's advantage comes from the built-in "house edge," which is a mathematical advantage that guarantees a profit over the long run.
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.