it's not difficult to do on a circuit, depending on how it's powered there will be a resistor to ground before the first input stage, change that and it changes the input resistance.
something like this before the first IC ... gives you three difference impedances.
(R1 and R27, and R2 and R26, can be combined - I just split them into two to take advantage of JLCPCB's cheaper SMD reisistors)
So in this case 20M with switch in middle or 3.33M or 1.4M in left or right positions...
(except there's other things going on in this circuit so it will end up 10M, 1.67M and 700k )