The Monster Exchange System is one of the greatest improvements to Puzzle and Dragons as it allows players to convert excess copies of Godfest Exclusives/Event cards for valuable Monsters. This process will consume 4-5 cards each time so it is not a decision that should be made lightly and should only be done when it will improve your current situation.
My general rule of thumb is to always keep at least 2 copies of any given card due to the fact that GungHo has been releasing numerous split evolutions and it is impossible to predict what will be released/buffed next. A great example of this is Valkyrie Ciel who went being an underwhelming card, to having two incredibly powerful evolutions where you would badly want each version.
In this article I will be outlining my thought process towards Monster Exchanging for Halloween Cotton, Elia, and Chakeol before it expires