Converting an NO reed switch to a NC with lowest leakage current
Fix the overall design so you don't have inconvenient issues like this at the lower levels.

The first obvious solution is to use a normally closed reed switch.  Second, why is the polarity important when it's going into a micro that can interpret high or low any way it wants to?  If you need to minimize power, have the micro sample the switch periodically, turning on the pullup or pulldown for a few 10s of &micro;s only around the test.  If this switch needs to be detected on a human time scale, then sampling every 50 ms will probably be good enough.

If you really really need to have a switch cause high impedance when closed and pull to ground when open, one FET and one resistor can do this:



This won't work well if there is any noise on the switch line, since the FET gate is at high impedance.  You can use a lower resistor, but that increases current too.  If you're going to reduce the resistor to have a few mA thru it when the switch is closed, you might as well substitute a NPN bipolar transistor for the N FET shown.