I've recently had cause to change a couple static routings on our Pix 501 Firewall. I've done this in the past, but each time, it has resulted in a period of time where the new static mapping doesn't take effect. Thanks to Jake on the PLUG Mailing list for pointing out to me the xlate command and usage here.

All you need to do is:
clear xlate

EDIT: Derek (same mailing list) suggests the following in order to only clear the one address, so that all the other existing customers are not interrupted.
clear xlate local <e.f.g.h>