Migration question

We would like to migrate from 9.21 to 10.0 on a new machine. We are also changing the storage regime. In testing, dbexport (without -ss) followed by dbimport on the new machine works just fine, but it does mean the system will be unavailable for about 5 hours when we do it for real. (In our test sessions, we do the dbexport from a research machine, as described below.) We can live with this, but I'm wondering if there is a way to reduce the downtime. In the past, I have been able to reduce or eliminate downtime when portions of the data needed to be exported by creating an archive (no downtime) and then restoring the archive on a twin research machine. From the research machine I could dbexport or onunload to my heart's content. No impact on users. I'm wondering if there is some clever way to reduce downtime for the Great Migration mentioned in first sentence. I can still create an archive and restore it on the research machine, and then dbexport/dbimport from research machine to the new machine. But, that leaves the final product on the new machine several hours out of sync with the production box (it would have continued to operate). Is there an alternative to the "brute force" way of just downing the system, dbexport/dbimport from old to new system (with the new dbspaces that are unrelated to the current production system). As I said, the downtime is feasible, just not desirable. DG