Elegant Jeweler's Backpack
Yes, the reason you can’t put an Exquisite jewel in is because its a level 78 item. Exquisite jewels can only be put in level 80 gear.
One of the crafted back items like the Elegent Jeweler’s Pack and a level 65 jewel is better than a normal Exotic backpiece with an Exquisite Jewel anyway, though. So its not really a bad thing. Just a bit confusing, perhaps.
There are no rules on putting an upgrade in and changing later. You can always slot a new upgrade into an item, it’ll just delete the old one.