How do I re-configure the "default gateway" (i.e., change it to the
new default gateway) on a SCO UNIX 5.0 machine so that I can continue
to VPN into the server through a new firewall?

I don't have a SCO box at my disposal currently, however, I believe
the command that is used in SCO is 'route'.

Usage for what you are trying to do should be:

route add default [gateway IP]

Obviously, however if you already have a default route you will have
to delete it, you should be able to do this with:

route del default [old-gateway IP]

You should be able to like the existing routes with by just typing
'route'

Sorry if it's a bit ambigious, but it's been a while since I used SCO.

See this page also (http://pcunix.com/SCOFAQ/scotec4.html#defaultroute)
as you will need to make changes to configuration files to make your
route reboot-proof.

Check these references:
http://www.experts-exchange.com/Operating_Systems/Unix/Unix_Setup/Q_10198288.html
http://pcunix.com/newtosco.html

I hope this works for you...

Cheers, 
sycophant-ga

On Linux, one would change the default gateway thus:

  # ip r d default
  # ip r a default via new.ip.address

I'm sure it is very different on SCO Unix, but there's my $0.02.

Noodle

Actually, SCO is almost exactly like earlier versions of Linux were
(before iproute2).