I understand that if I have only one extra backpack (not permanent) and get a second one a day before the first one expires, I'll have two backpacks for a day and then be down to one.
What happens if I already have the full 3 backpack slots, and one is about to expire? I would like to renew the one that is about to expire without letting it disappear first because there are items in it. Will activating a new backpack slot item work? Will the new duration overwrite the old duration that was closest to expiry?
Your items in that backpack will not be deleted. Once the expiration time is done. Just click the extra backpack, and you are able to use it again.
You cannot use the item in an expire backpack. Example if the mech is in the expire bag, it will not activate unless you move it to a backpack that is not expire or your main backpack.