Today I played Agricola through an online portal for the first time and it seemed to think that this upgrade incurred a cost of two clay (possibly equal to the difference in cost of the two cards). I can't find any reference to that in the rules, and it's not how I've seen anyone play the game before. As far as I can tell, the cost on the Cooking Hearth is either/or, you return a Fireplace or pay 4 clay, never [partially] both.
There should be a Word Games Subdomain, or at least a Word Games Forum!
The cost is that it took an extra action to do it.
1) So you can build a fireplace for one action and 2 clay. 2) Or you can build a cooking hearth for one action and 4 clay. 3) Or you can build a cooking hearth for two actions and 2 clay. You do #1 first, and then take one more action.