Update terrain object with vegetation applied

Hi all,
I have a number of large terrain surfaces that I have imported via datasmith, I would like to paint trees and vegetation, then when the design is updated, I want to replace the object, and keep the planting. I have tested, and managed to update the surface, however the planting does not update. My master file is in max 2018, latest export plugin. The surfaces have to come from max and not created via the landscape tools.
many thanks!

are you reimporting same datasmith file.