Allow players to downgrade champions to lower ranks and return catalysts, gold, and iso to allow players to upgrade more desired champions.
What you're asking for actually already exists: it is called an RDT, or a rank down ticket. These were used in the past as high value rewards and sometimes to address issues with champions that were changed radically after launch. However, it is very rare to see them, these days, and they are never going to be a generally accessible thing. Reason being, it is precisely because it allows players to recover spent resources and reuse them. There are lots of reasons why this is a bad idea, the two biggest are a) this trivializes rank up decisions because players can always undo them, and b) this gives too much of an advantage to veteran players who have massive storehouses of rank up resources they could recover and apply to newer or higher rarity champions.
And before you or anyone else says it, yes, one RDT is not game breaking, which is why we on very rare occasions see one. But when you give one, someone asks why not two, then three, then why not just make them available. RDTs are a drug players too easily get addicted to. It is just not worth it to release them in any small quantities just to fight the fight over where the line should be drawn.
Your suggestion falls into the category of wars long fought and won (or lost, depending on which side you're own).
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And before you or anyone else says it, yes, one RDT is not game breaking, which is why we on very rare occasions see one. But when you give one, someone asks why not two, then three, then why not just make them available. RDTs are a drug players too easily get addicted to. It is just not worth it to release them in any small quantities just to fight the fight over where the line should be drawn.
Your suggestion falls into the category of wars long fought and won (or lost, depending on which side you're own).