I fitted a pair of front panel toggle switches to our SCA N72's and J99's so I can easily change both the mic input transformer impedance and the output transformer loading without having to remove the modules to change the jumpers on the PCB's. (Haven't labelled them yet)
I'm finding I often prefer the 300 ohm setting on the mic input transformer even with our LDC's as it warms and fattens up the lows and is usually a little less sibilant than the 1200 ohm setting and can provide more of a vintage vibe if that's what you're looking for.
It also allows the singer to back off the mic without the low end getting too thin.
For those who haven't tried the 300 ohm setting I can recommend it especially if you need to take the edge off a bright condenser and thicken it up a little.
Don't worry too much about the 1 to 10 mic input impedance 'rule' just follow what sounds best.
Would love to know how you performed this modification.
I know I'm resurrecting a dead thread, but essentially you simply need to switch in an appropriate value resistor across the primary of the input transformer, such that the combined parallel value with its existing input impedance is the overall value you're after (in this case 300 ohms). If the existing input impedance is 1200 ohms then the parallel resistor would need to be 400 ohms (390 ohms is the nearest preferred value, and close enough).
Peterson of DIYRE posted up a handy way to do this via an external box, for fully adjustable input impedance with any preamp here, which should give you the general idea: