No .. you cannot .. you can do v7 to v8 and the process will migrate the catalog and all other structures to the latest version ,,
But going from a higher version to lower is not supported (and doubt, will ever be supported)
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Because backup/restore to previous version is not supported, you will probably need to do something like this:
1. db2look - to get database objects (primary keys, foreign keys, etc) run "db2look /?" for more help.
2. export tables
3. import or load tables