It seems to be a good idea; one exchange per day is okay.
Did you suggest that in an exchange a random rare card was won but, with a 100% winning rate? Because, noone would appreciate an opportunity where one loses 6 rares for a chance of winning, I suppose.
Should the possible rares include Nymphs and Marks too (with a really low chance of course)? - If not, perhaps there should be a chance for winning an upgraded rare (even if you exchanged unupgraded cards). Alternatively, only exchanging upgraded rares give you a chance for Nymphs and Marks? Other opinions about this?
No one wants to lose 6 rares for only 800 electrum either :/
About nymphs/marks, ultra-rares should stay ultra-rare.
Yes, obviously it should be a guaranteed win rate for a new rare.
I also agree, the level of rarity on various rares should be maintained.
Its ultimately up to Zanz, but I think that at the very least you should only be able to get an ultra-rare if you sacrifice ultra-rares. E.g. to get a new nymph you would have to sacrifice 6 nymphs... or just leave nymphs and marks out altogether. (There should be some incentive to visit oracle daily and play in tournaments after all)
I mainly just want a way to, for instance, to trade in the bunch of owl's eyes I seem to get ad-nauseum for say, a chance at a new pulverizer, or maybe even a shard.
As for getting upgraded cards, I would think that if you sacrifice 6 upped cards your spin should give an upped rare. If you sacrifice unupped cards, you get an unupped rare.
A feature like this will be more important as more rares get added to the game.