A look at the movement of lines on the western front, the Germans went back in early 1917 and then in early 1918 conducted an offensive that is considered successful, but really only re-occupied what they had given up. WI they never went back in the first place? What would happen elsewhere if the 13 divisions freed up by the new line were left in place for 12 months? How would the March 1918 offensive go if it was launched from the pre Hindenburg line positions?
All hangs on whether they can hold out against the British offensive at Arras. OTL, even on the new line, Ludendorff got very jittery that the British were going to break through, and Hindenburg had to calm him down. Their old, more advanced line might have been harder to defend.
Of course, much depends on what motivates the Germans to cancel the withdrawal to the HL. If, say, the Russian Revolution has come a few months early, they may be able to transfer enough troops west to be confident of holding out where they are; but that of course would cause major butterflies in other matters too.