Sand dunes are ubiquitous in nature, and are found in abundance on Earth and other planetary environments. One of the most common types are crescent-shaped dunes known as barchans, whose mid-line could be assumed to behave as 2D dunes. In this work, we (i) compare the morphology of the mid-line of 3D barchans with 2D dunes; and (ii) track the evolution of 3D barchan and 2D dunes under bi-modal changes in the flow direction. We performed experiments in a 2D flume on 2D dunes and Euler-Lagrange simulations of 3D bedforms. The reversal experiments start with an initial heap deforming into a steady-state dune, which is then perturbed by reversing the flow, resulting in an inverted dune. We show that during the reversal the grains on the lee side immediately climb back onto the dune while its internal part and toe remain static, forming a new lee face on the previous stoss slope of varying angle. We determine that (i) the characteristic time for the development of 2D dunes scales identically with that for 3D barchans, (ii) that the time for dune reversal is twice the time necessary to develop an initial heap to steady-state, and (iii) that a considerable part of grains remain static during the entire process. Our findings reveal the mechanisms for dune reversal, and highlight that numerical computations of 2D barchans, which are more feasible in geophysical scales, predict realistic outcomes for the relevant time-scales.