This might not help you ... but 10gR2 has a concept of flashback
database
and creating restore points. Before you start your "transaction", you
can create
a restore point which if time comes you can restore the whole database
back
to using this feature.
10gR1 also has the concept of flashback database.

That is the closest you can get to regarding undoing the upgrade.
Works on a database level though ..

see (oracle docs at tahiti.oracle.com ...):
http://download-east.oracle.com/docs...7.htm#sthref74

Anurag