You should be able to modify it.
Make sure it is not connected to a library (rslib), or disconnect it.
Make sure to detach any object attached to the mechanism (e.g is the robot mounted on the gantry ?)
Also if it was built with e.g an older RS version it might block for modification.
Anyway if you just need to raise the mechanism by 100mm : it is much simpler to move the current gantry and update the calibration or baseframe.