ACL switching of parent to sibling...

Does anyone know a clever way of using an upstream proxy as a parent
during work hours and a sibling otherwise? I've tried various ACL's and
haven't found a way yet...

The problem comes from the fact that we use the parent only during the
day because we need more throughput then, but we pay for parenting
traffic per meg. During the night we have enough bandwidth but
obviously we'd still like to do sibling lookups from the upstream.
Right now I have to switch the upstream off after hours because I can't
stop my cache using it as a parent when its in use.

Cheers
Brad Meier

--
Systems Software
University of Cape Town