I still feel like the Theros Prerelease was a couple of years ago, instead of the better part of a decade, and I still feel like the same person I was in 2010 or 2016, even though all the circumstances of my life have changed completely
First, while a double-faced permanent is on the battlefield with its back face up, its mana value is calculated using the mana cost of its front face
(Choose any number of permanents and/or players, then give each another counter of each kind already there.) You can choose any permanent that has a counter, including ones controlled by opponents
Your best precaution against these factors is to get a storage unit that can maintain consistent temperature conditions
Greymond, Avacyns Stalwart, for example, is worth over $47 at time of writing