Do you still have the old keychain? a simple login into a admin account (or different) and moving it back were it should be and replacing should get it.
if that doesn't work and If you have os 9 boot into it and move it. back Os 9 will have no problems as it doesn't understand keychain and will move it without a problem.
A quick help because i know you might ask. The keychain should be in /Users/YOU/Library/Keychains