I believe you can't replace a unique upgrade with itself, because you can't even play the card from your hand.
"There cannot be more than one copy of a unique character on a team, and a player cannot play a unique support or unique upgrade if they already have another copy of that card in play ."
Not only that, look at how you're supposed to play upgrades, you put it in play first, then replace one if you want, then you pay the cost..so...
"Playing an Upgrade: The player chooses and attaches the upgrade to a character by placing it next to or below that character. Before paying the cost to play an upgrade, the player can choose to replace an upgrade that is already on the chosen character. The cost to play the new upgrade is decreased by the cost of the old upgrade, and the old upgrade is discarded when the new one comes into play. If the old upgrade costs equal to or more than the new one, then the new upgrade is free."
that's what i thought at first until others convinced me otherwise
i'd like to see an official ruling on this topic