I'm exporting an RBD collision from Houdini into UE4, how do I replace the collision geometry of the groundplane in Houdini with the terrain i've created in UE4? (this terrain was actually created in Houdini and brought in as an HDA)

I'm not 100% sure I properly understood your question, but maybe this can help: couldn't you just use the same terrain you created earlier in Houdini (I'm guessing you used heightfield tools) as a static collision object in your RBD sim?(attached is a hip file example)

Or have you altered the terrain in UE4, and now want to use that new altered terrain in the RBD sim?

If you have edited the terrain in Unreal, afterwards and need to get those changes back into Houdini fro the sim:

With Houdini Engine you could plug a Landscape in Unreal into an HDA. Once you have the data in Houdini you can do with that, whatever you want.

https://www.sidefx.com/docs/unreal/_landscapes.html [www.sidefx.com]