Most people do not migrate their apps as the V5 apps will continue to work and there isn't a need to maintain them.
The way the app migration from V5 to V7 works is that you create a new app in V7, migrate in your V5 images, and publish the V7 app. When you migrate in the V5 images, it will also update the V5 app to say that the app moved and provides a link to the V7 app. So people who have a link to the old app, or have the old app installed on their device will receive the message, click on the link and then will be prompted to install the new V7 app on their device. The old V5 app will no longer work and just forward people to the V7 app.
To convert, when you create a new app in V7, on the gallery section, right next to the trash can on the right is an icon that shows an arrow going into a box.
If you click on that icon, it will open a dialog that will give you information on how to migrate your apps.
Apps need to be migrated one at a time. There currently is not a bulk migration capability.
If you need a list of your version 5 apps, just go to http://v5.myphotoapp.com/old/directory.html and login with your version 5 id and password to see all of your version 5 apps.
Note: If you do not see the migration icon, then that means that your V7 account email address is different than your V5 account email address so MyPhotoApp doesn't think you were a V5 user. In that case, you will need to recreate your apps from scratch in V7.