Open map in GE
goto Windows(C) -Programme Files(x86)--Farming Simulator 2019 --data- maps -trees and import the releveant i3d into your map (i.e. birch.i3d)
in GE --Edit --Replace -- a Replace box will open
Select the tree you want to replace and and select "Load" in the "Target Section" i.e birch_stage_3
Select from the imported birch.i3d the tree you want to use to replace the original i.e birch_stage_4 (or any other size for that matter)
Select "Load" in the "Replace with" section
you should now have , "Target" --the tree you are replacing and "Replace With" the tree you are using as your replacement.
Select Replace All
Every tree of the type you selected on the map as the Target has now been replaced with the tree you selected in Replace With retaining original scale and translations.
Save and rezip map.