Yeah. If you want to get your original height back after changing the Z-scale, you can export your heightmap to for example Krita och Photoshop. Add a Multiply layer that is fully black and opacity set to 50%. (if your Z-scale is 200 that is) This will scale your heightmap down by half, which will counter the new scale you made in UE4.
Note however that it is likely that your terrain will be moved down… Because the centerpoint is at 50% grey. I guess you can start by restoring the height. Then do a second pass in PS or Krita, but do an add layer with fully white, but set to 25%. (this should move everything up to the same place it was before I believe)
Remember to save your heightmap in 16 bit to not loose any information. Most images are stored as 8 bit, so double check the bitmap precision before saving the png.