Please find attached the Surface Assignment script file we've made for you. After unpacking you only need to drag and drop it into the 3ds Max viewport.
Its UI is quite simple. To select the "new" surface please Pick Surface button. You can select Forest object(s) manually or if wanted you can select all Forest object in your scene by Select All Forest objects button. By hitting Assign all selected Forest object will now use the picked surface.
Hope, you'll find it useful and it'll speed up your workflow.