[j-nsp] How to upgrade M20 RE's with least downtime

[Bill Petrisko]

I need to upgrade the junos on an M20 with redundant RE's.

What is the practical method of doing this, in order to have
the least amount of downtime?  Are there any issues with 
upgrading one RE at a time, and performing a manual switchover
between upgrades so that the router is not down for the
full install process?

I am thinking either...

a) Upgrade (backup) RE1, reboot it, etc
b) Force switchover to RE1
c) Upgrade RE0
d) Let it run on RE1 

Or...

a) Force switchover to (backup) RE1
b) Upgrade RE0, reboot it, etc
c) Force switchover to RE0
d) Upgrade RE1, reboot it, etc

I just want to make sure that one of the above is an acceptable
or a customary way of doing it -- also that nothing bad will 
happen if I have two RE's running at different version levels
for a brief time.  

bill