a toggle between a 250k or 500k pot/resister.
a variable resistor to ground at the input to the buffer.
A (variable) resistor to ground, parallel to your volume pot?
As incredible as it may sound you already have exactly that: the tone control on your guitar...
Turning your tone control to 5 (on a linear pot) would sound exactly the same as changing your 1M Ohm volume pot to a 500k Ohm pot.
Turn it to 2,5 and it'll sound like a 250k vol pot, although at that point you'll start to hear the effect of the cap on the tone pot.
It should be in series with a resistor so you can't have a situation where you can wipe out all the sound.
Which is why we have a cap on the tone pot?
The resistance in series with the inductance of the guitar cuts the treble.
The (additional) resistive load lowers the amplitude (Q) of the resonance peak (usually in the treble range).
This is what the tone control on a guitar does.
Additional capacitive load lowers the frequency of the resonance peak.