Switching over from Xtal to variable oscillator with minimal hardware?
If the microprocessor contains its own oscillator, requiring only an external crystal (not an oscillator module), it isn't really necessary to disconnect the crystal. It has a high impedance at the low frequency of your slow-speed oscillator and does not affect its operation.

So, a simple "volume control with on-off switch" could be used as follows:







simulate this circuit &ndash; Schematic created using CircuitLab

When the switch is open, the crystal oscillator works normally, as long as the wiring between it and the switch does not introduce too much stray capacitance. When the switch is closed, the oscillator easily overpowers the crystal, and the frequency of operation can be adjusted by the potentiometer.

If the wire between the switch and the crystal cannot be kept short, use the switch to control a relay that is closer to the circuit.