How to switch between accurate current sources
You are making your life unnecessarily difficult. Instead of switching both sense resistors, 







simulate this circuit &ndash; Schematic created using CircuitLab

Also, you should look carefully at your MOSFET's Rds(on) rating. Usually, the rating is given at large currents. Since you are only dealing with 100 mA max, you can probably get away with a cheaper FET.