You can access this tool from the following:
This command allows an existing complex alignment to be partially redefined over a range, substituting in new horizontal geometry.
The command preserves the original Alignment name, allowing all rules built off the original geometry to update in line with the new geometry. Complex redefine affects all rules relating to the alignment meaning other geometry, superelevation, corridors, terrains, civil cells etcetera need to reprocess. The vertical geometry from the original alignment is preserved and carried forward to the newly formed alignment. The segment from the original alignment that is replaced is retained in the model and is allocated a new name.
Note rules over reference files will need to be opened to initiate the update.