The relations your talking about are indeed not kept if you delete them. Theoretically, it's not need to delete them since they have a date linked to them. So you if the owner changes, you'll have a start date for both relations.
As for spare devices, easier method is to use asset states. There is a built-in 'Spare' state. Only thing to be aware of is that a lot of reports have a condition in them which will only display assets with the Active state.
