Here's what I did in my mod:
The Swamp terrain is in fact a combination of a base Terrain type, a terrain Feature and a special Resource. So firstly I divided the Swamps into one variety with the Feature and one variety without it. The Feature part can also be drainaged by simply "Cutting" it down, as if it were Forest or Jungle (the other types of Feature, along with Flood Plain). This is an industrial age feature and requires Railroad (for steam powered pumps).
Also, I permitted some Improvements on Swamp tiles, depending on if the Feature part was still there. Most units can also move through Swamp terrain (not siege units among others) but they receive damage every turn. A Road (or Railroad) effectively takes care of this as units mostly won't stand on the Terrain in between turns. And you can always remove the Feature layer later on (see above).
If you get your mod to work I'd probably use it as a base for my own modding, so please keep the rest of us posted on the development. I'm not as sure about removing a Swamp from a tile completely though, and absolutely not turn it into Grassland! There's a lot of Swamp in Siberia, and drainaging all of that would make the region anything but the Taiga it should be!
So I think that the Swamp terrain shouldn't be removable entirely, but it shouldn't be completely impassable to the majority of units, and it should be workable and improvable. It would probably be some of the worst tiles in a city's BFC and it would probably be better to run specialists instead of working them, even with improvements. I have enabled Forest Preserves on them though (renamed Nature Preserves) and since the other kind is forested you can use them for Lumber Mills later on. Also, you could build riverside Cottages on Swamp tiles but they'd have to be supported by other tiles for
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