Have you looked at SharePlex? We did a proof of concept on our crm database (8.1.7 on HP/UX to 9.2 on Linux) a few months ago and the actual downtime was in the seconds.

After an export, it reads the redo from the source and collects dml until the target completes it's import .. then it catches up with the source by applying the cached dml .. you shutdown the source, complete the sync, then point the users to the target. Then you can back-port the dml from the target back to the source, just incase something really goes wrong. It's really cool stuff.

Best of luck on your migration...