db "hot switch"

Hi all,

I want to actualize my postgresql version stopping the server
the minimum time possible. I was thinking in turn the database to
read-only state (selects will work normally), make a dump, restore the
dump in the new installation and stop the server only the time needed to
stop old postmaster and load the new one. The problem is that i don't
know how to set readonly state. In mysql for example i left only select
privilege on all users (first i backup users), but in pgsql i can't
revoke or grant privileges to all users on *all databases* easily and i
don't know how to backup only users without a pg_dumpall.

¿Any suggestions? Thnx in advance

----------------------------
Pablo Varasa Paredes
Dept. de sistemas
Prisacom - Grupo Prisa
mailto:pvarasa(at)prisacom(dot)com
C/ Ribera del Sena s/n 28042 MADRID