The reduction is made to prevent “banking”. If one were able to cancel an upgrade after a week, getting all the gold or elixir back, then those resources (which could be in the millions) would have remained completely hidden from opponents during that period of time. With this technique, players could keep loot from invaders for as long as they want without any consequence, simply getting the resources back whenever they want them. That's why only half of an upgrade's cost is returned if you cancel.

Additionally, we’ve added layers of confirmation to ensure that when a player makes an upgrade, they are asked again if they are sure it’s what they want. The same goes for when you cancel an upgrade.

If you use gems for an upgrade, they are immediately converted into the required resource type (gold/elixir/dark elixir) to start the upgrade. Because resources can't be turned back into gems also the ref