Another approach is Points > Point Utilities > Elevations for Points
in the dialog box pick the Differential option; then enter either a range of points, a point group, or Screen Pick; then enter your elevation shift in the box; hit OK
With either method, none of your 3d polylines or text labels with elevations will update (if you have any) to the new elevations. If you have not done any finish drafting, just run Field-to-Finish. If you have finish drafting edits you don’t want to lose, you will have to shift your 3d polylines by the same elevation shift using the basic Move command. I use a base point of 0,0,0 and move the lines to point 0,0,ElevationShift. You will also have to update your surface model, if you have one, and re-draw the contours.