Migrating activities to activity sets should not be done in portal upgrade. It should be done in plugins because:
1. social.activity.sets.enabled=false is set by default. Most people will not use activity sets for now as there is no portal use case. Makes no sense to update all previous activities, but not the new ones.
2. For 6.2 users who install SO on top of their existing portal instance, the existing activities wont get migrated.