Depending on the calculator, the OS can do that on its own. My overclocked 83PSE has 4 speed modes, all of which can be accessed by sending different numbers to port 20h. It may be difficult to control with a microcontroller on other calculators because the clock speed changes with the capacitance, which would be difficult to change from a microcontroller.

The calculator already does that to give the four different CPU speeds, except it uses resistors instead of capacitors. The SMT soldering required to do what you suggest gave KermM a run for his money, and he has a master's degree in this stuff. If you desolder the capacitor and can't get a replacement installed, your calculator's CPU will not function. You're dealing with components with sub-millimeter sizes. You need special equipment and special skills to avoid breaking things.

as i have a 84se, unless someone finds a way to do so with it explicitly,  i wont be tryinng anything at all, but i do have a spare that i botched the ribbon cable on while debugging my speakers (crule iorny is they work almost flawlessly now) that i can experiment on using the tilp screen shot to veiw progress.