As of Inventor 2008, when you open a file and save a file in Inventor the file will be migrated to the new version.
The file is not migrated if you do not save the file.
On demand migration requires you to open and save the file for it to be migrated to the new version regardless of whether make a change to the file.
Important to keep in mind:
There is a Save Application Option that, when enabled, will prompt you to save and migrate when you open and close old Inventor files. See Figure 1 
.
Figure 1
Prompt to save for migration
- When you open and then attempt to close a file that has not been migrated, you are asked if you want to save. If set, you will be asked if you want to save and migrate on open, even though you have not made a design change. By default, this option is off and we recommend you leave it off.
Figure 2 - Prompt dialog
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Note: This Application Option is not used if you use the Task Scheduler or the Inventor application menu > Manage > Migrate command.
How to check migration status
Inventor can check file migration status via iProperties.
Step 1: Right click one file to display the context menu and click iProperties button to open Inventor file iProperties dialog. See Figure3. 
Figure 3
Step 2: In the iProperties tab, you can find the version information. In the Needs Migrating box, Yes indicates the file is not yet migrated.