This only makes sense for rares/ultrarares. A regular bazaar card you can just buy an unupgraded one for 5-100 electrum or so, depending on what it is. For a rare like, say, a shard or weapon, it could make a small amount of sense to downgrade a 7th upped rare to complete a set of unupped ones, although the odds of ever actually needing to do so are extremely small. (for me, the only time this would have helped is with SoRs, as I have sold extra upped ones but have only 4 unupped ones, and even then I'd have saved my electrum for other uses)
The place this would make the most sense is with nymphs and marks. Right now, people rarely upgrade those because the majority of pvp events are unupgraded, and since they are hard to get, you can't just rely on getting another one anytime soon if you upgrade one. For example, I have one upped nymph (aether) because I upgraded it before I had ever entered a pvp event and so I didn't know they were usually unupgraded; I figured it made sense since the upped aether nymph is strictly better than the unupgraded one. I also had a red nymph at the same time, and I didn't upgrade it at the time, because the upped one is not strictly better, and I knew I wouldn't be able to undo it. (I think the red one is the only one that's not strictly better upped, but I'd have to look at the list)
