migration, upgrade, enhancement

Migration of a software application from one platform (32bit OS1 on xxx hardware) to another platform (64bit OS2 on yyy hardware)

Upgrade of a software application involves upgrading the software from a lower version to a higher version (like XX version of database to YY version of database)

Both the above activities are technical and no business benefit is directly attributed to this type of activity on the software applications.

An application enhancement will involve providing a new feature or functionality to the end-user of the application. Sometimes when the application is majorly enhanced, the new enhanced application can get migrated to a new platform and/or the underlying technology stack upgraded to higher version.

The key question I am dealing in here is:
When there is no "enhancement" involved, purely a migration and/or upgrade is planned:
a. is it good to do both together in a big bang? OR
b. is it good to migrate first to the new platform and upgrade the software later? OR
c. is is good to upgrade the software first and then migrate to new platform?

a. One can do it together but if there is an issue it is difficult to find the cause of the problem due to new platform or new software version.... each can point fingers at the other....
b. is a good choice if the platform migration is more complex than software upgrade.
c. is a good choice if the software is more stable on the higher version and if any specific platform related issues are fixed in the higher version.

If the software application has multiple tiers with database, application server etc., one should carefully choose the overall path of migrate and upgrade. Making sure the impact on the business SLAs is minimized; It is also important to have a very good "back-out" plan, if something badly goes wrong!!