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One crucial requirement that I would strongly recommend is that you create a duplicate backup site first. This is detailed in the faq section, if not contact me. This backup will provide a test-bed for your upgrade procedure.
Do the upgrade on this backup first, test all the modules and system operations and only then when you are staisfied that you have navigated any problems, proceed to replicate the process on your production site.
It still beats me why users do upgrades on production sites without a trial run on a backup first.
You want peace of mind? use a backup for all new modules, upgrades themes and experiments.
I personaly have three backups that take all the load of experimentation.
Doesn't that make sense!
Richard