(a) First step, before going for any up gradation, you should take full backup of the existing databases including system databases. If anything goes wrong you should have a full backup to fall upon
(b) Run Upgrade advisor : You must run this tool before going for up-gradation. This tool will analyze the existing database in lower version and suggest the potential problems , which may be because of the feature is removed or deprecated in the newer version. Once the upgrade advisor projected any critical problem , you may address the problem before going for up-gradation.
(c) Once you found that there is no major issues reported by Upgrade Advisor, you can plan your up-gradation. You must document each and every step you follow. Also ensure that you have a rollback plan incase anything goes wrong. You must have a proper testing script as well.
(d) You have two choices in upgrade.
In Place upgrade : When you upgrade an earlier SQL Server release in-place through the install upgrade process, all existing application connections remain the same because the server and server instance do not change. This approach requires a more thorough fallback plan and testing. By performing an in-place upgrade, logins and users remain in-sync, database connections remain the same for applications, and SQL Agent jobs and other functionality is concurrently upgraded during the installation. Note that several features, such as log shipping, replication, and cluster environments, have special upgrade considerations.
Side By Side : This method is more or less manual process hence you have full control over the process. You can test the upgrade (migration) process by running parallel system , test it and once proven bring it online. But the disadvantage here is, you have to have additional resource (Hardware/licenses ) . Side by side will be always better since you have the existing instance intact. But at time it may not be feasible because of hardware constraint or Instance Name (like the application demands Default Instance. I would say this method is more clean and controllable.
(e) Once you have upgraded test the system with the application with full load before going online.
Migrating Database in Side by Side Upgrade.
(a) If you have chosen side by side method, after installation of new version of sql server you must migrate the user database from older version sql server instance. There are three options available here.
(a) Backup /Restore : Best and easy method. Benefit is source database can still be online , no downtime
(b) Detach / Attach : You have down time at the source but easy method.
(c) Copy Database Wizard : This tool internally does detach/attach only.
Migration of SQL Logins
This is one of the major disadvantage of Side by Side up-gradation. You must transfer the SQL Logins explicitly. You must transfer the login with the password. Microsoft has provided the script for that. How to Transfer SQL Logins
Migration of SQL Scheduled Jobs
Simple , just need to create the script from source and run that script in target server.
Migration of SSIS Packages
You can use save as option of SSIS package . You can open the package and save the package as file system and then migrate.