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1. Yes and this always should be done before do an upgrade :) backup your all entire site and all database.. because if something happen after upgrade or the upgrade got stuck, you can revert it back into old version :)
2. Yes, all your customization related with jomsocial will be overwrite with new jomsocial script.. :)
3. To know about it, you must did and upgrade first since we don't have any change log now (because this is not stable version)
4. I'm very very sorry for this, our support developer doesn't provide any migration, or upgrade help :) you can search how to upgrade from our wiki page or from our forum :)