I would suggest adding rowkey in Apps8 version before the upgrade. So that you have it done already and do not need to bother about that during go live.
Maybe someone who has actually done this can guide you about the steps.
There is a script to be able to run this incrementally according to the documentation. More info in link below, if you have not found it already:
https://docs.ifs.com/techdocs/foundation1/020_installation/040_upgrading_system/020_preparing_upgrade/020_installation_roadmap/hidden_itd/030_fndbas_rowkey_details.htm#BackgroundJob
Thanks a lot @Tomas Ruderfelt
Great and very helpful reply
Regards
Pankaj