Best way to modify /etc/ppp/ppp.secrets on the fly?

[Brett Glass]

I need to write a program or script that modifies /etc/ppp/ppp.secrets on the fly to add, change, and remove passwords. One thing I do NOT want is for replacement of the file to interfere with a login that's occurring at the same time. What's the best way to slip a new version of the file in without messing up an instance of userland PPP that might check it at just the wrong moment?

--Brett Glass