swamp irrigation

[Wahazar]

In my opinion, swamp irrigation should result in irrigated swamp, not altered to grassland, such feature is annoying due to lost of special resources (there is long way to recover them, and you need to remember where they were located).
Of course, possibility of transforming swamps before engineers era is very nice, but why not by additional "Transform_Possible" effected bonded to "Terrain", "Swamp", "Local" ?