Reference: Change Formation, Page 16.

Units can add or reduce the depth of their formation by a single rank at a penalty of half a move, or by two ranks if they remain stationary. A rear rank still counts as a rank even if it has only one model in it, although no hand-to-hand combat benefit is conferred unless it contains at least four models.

Below, here is a unit of 15 models in 3 ranks, and the same unit redeployed into 2 and 4 ranks. In both cases the centre front model remains in place (the ideal position for the unit's leader) whilst the length of the line is reduced or expanded as evenly as possible about this centre point. Often, as in this case, you will have to decide to move models from the left or right hand side because the number of models in the line shifts from odd to even, but you must still endeavour to keep the centre front rank model as close to the centre as possible.

The easiest way to reduce a rank is to remove an entire rank from the formation, and then rearrange the models evenly to the sides.

When adding a rank remove a file from one end of the formation and line it up at the back, then, if there is sufficient room, remove the file from the other side of the formation and line it up - and so on until there is no room left to rearrange a whole file.

When adding or reducing ranks you must be careful to ensure there is room for the models to fit in their new formation. If space is blocked by other models or impassable terrain or buildings, if the edges of a line should creep into difficult terrain the unit does not already occupy, or if models must move over an obstacle, then the formation change is not permitted.