Say the alignment at the beginning of the scheme was required to be extended by 20m - the section originally at chainage 0m now is at chainage 20m - this might be a problem particularly if drawings had already been issued with the sections at the original chainages. By default the sample lines and hence the sections are locked to the alignment and will adjust to suit the edited alignment. See screen grabs below.
Alignment original chainage and sample lines
Alignment new chainage and sample lines
You can unlock the sample lines from the alignment which will get around this issue. Select one of the sample lines in plan, right click and select similar, right click again and select properties. You can unlock the sample lines here by setting the value to FALSE.
However, this means that any changes to the alignment horizontal geometry will not be reflected in the sections and so you may still want the sections to be locked to the alignment but hold their original chainage. What we need to do is modify the alignment reference point.
Keep your sections locked to the alignment, extend the alignment start chainage by the desired value. Select the alignment and in alignment properties on the Station Control tab click the button to set the reference point for the alignment making sure that the Station is set to zero and pick the start of the original alignment in your drawing. See below:
Now we have an extended alignment but with sections at the original chainages that are still linked to the alignment should we need to make changes later!