Thank you for your answer.
From the pictures, it looks like the new fort has pointy stockade, while the "old" forts are more round or smooth at the top of the stockade. Also the "murder holes" in the middle of the walls are different. So even if they physically fit, it will be quite obvious that they are not the same type.
However, I guess this can be more or less camouflaged by having a tower or something between new and "old" walls - if they both are System X.
Time will show