In Alchemy, the rules for what kinds of cards can replace a trashed card are governed by the following, somewhat ambiguous wording:
Adding coins to a cost doesn't affect [potion] being in the cost or not. If [potion] was in the cost, it still is; if it wasn't, it still isn't.
The rules then present an example with Remodel card. However, the Remodel card says:
Gain a card costing up to two more . . .
The Governor card wording is:
Gain a card costing exactly one (or two [if you're the player playing the card]) more.
So, for the player who played the Governor card, it is clear from the remodel example that a card costing exactly 2 coins must be replaced by a card costing exactly 4 coins with no potion.
However, what about a card costing exactly 2 coins and a potion? If trashed, must it be replaced with a card costing exactly 4 coins and a potion? Or could it also be replaced by a card costing exactly 4 coins (but with no potion)?