Any time a ZenPack is installed Zenoss looks in the ZenPack's migrate directory for steps whose version is greater than or equal to the version of the ZenPack being installed. Migrate steps are classes that subclass ZenModel.ZenPack.ZenPackMigration. This mechanism allows zenpacks to modify items in the object database that were created by previous versions of the ZenPack and need updating. The ZenPacks.zenoss.HttpMonitor Core ZenPack includes good examples of how migrate steps are written.