Answer
Do this kind of heavy, long-running schema preparation before the upgrade window itself, not during it — specifically, add ROWKEY to your largest tables while still on the source release (Apps8), using IFS's documented incremental ROWKEY-addition script, which lets the work be run in smaller batches ahead of time rather than as one long blocking operation during the actual upgrade/go-live. Getting this kind of large-table schema change out of the way in advance is a general upgrade-planning principle worth applying to any similarly heavy structural change, not just ROWKEY specifically — it keeps the actual go-live window shorter and lower-risk.