As a production DBA for over seven years with a large-ish financial services company, I've seen more than my share of upgrades. Over the years I've added and modified my upgrade/migration checklists. The latest version, for SQL Server 2014, is available here:
Upgrading to SQL Server 2014: A Dozen Things to Check
The post covers a variety of topics:
- Hardware
- Compatibility checks
- Data integrity
- Upgrade versus migration
- Ensuring reliable performance
Have a look and let me know if there is something you have on your checklist that you feel should be on mine.