The schematic for the original (I have no idea if Dunlop used the identical circuit beyond the status LED) can be found here: http://www.generalguitargadgets.com/pdf/ggg_p100_sc.pdf
The 4-position switch does two things at the same time, that are similar to what the Color switch on a Small Stone does. Both switches adjust how much resonance or feedback there is at the same time as adjusting the sweep width. So, of the 4 positions you can have wider sweep with and without resonance, and a narrower sweep with and without resonance.
The Phase100 has 10 phase shift stages, 4 that are fixed (IC5b, IC6a, IC3b, and IC2a), and 6 that are swept by the modulation/LFO circuit (IC6b through IC1a). Look closely and you'll see that the output of IC2a goes to the switch and from the switch through a 15k resistor to the input of the 3rd phase-shift stage (IC5b). The resistor is mixing the output of the second last stage back into a point earlier in the chain, and that's what produces the resonance.
The amount of resonance is set by the value of the resistor. We're going to changethat from a fixed resistance to a variable one, to produce variable resonance/Q/feedback. Ideally, the feedback should never be mixed in at a greater amplitude than the input signal for that stage. Since the input and feedback resistor in each stage is 10k, the feedback signal cannot pass through a resistance less than 10k or else you risk oscillation/howling. However, there are many degrees of resonance in between 15k and open circuit (the other switch position), and it is a reasonable mod to try and get them. Ideally, you'd want something that is around 11k as the absolute minimum value in place of the 15k unit, to prevent any howling. Or you can use a 10k value that measures a little over 10k, and let the added control do the heavy lifting. A pot value of 25k or 50k is a good choice, though you may find that you want 100k if you wish to have negligible resonance (and 100k will be far enough away from 15k to be almost like open circuit).
The other thing you can do is remove the 20k Centre Freq trimpot from the board, and substitute a 20k or 25k panel-mount pot to dial in the "zone" covered by the nothces.