You can manually alter any unit's selection rank on the fly with an option toggle. https://zero-k.info/mediawiki/Selection#Selection_Rank
You might misunderstand. Default behavior is to drag a box and select all combat units if any exist in box selection(tier 3). This new option allows you to hold alt while doing a box select, and it 'inverts' the selection of units, so now if any combat units exist in the box select, they are NOT selected and all lower priority units (rank 1 and 2) are selected. This allows a player to easily do a not
action with a mouse and 1 keyboard modifier. Of course, all the default behavior has been unmodified, so this patch shouldn't affect any pre-patch selection behavior. Besides that, I don't understand any new reason to be shifting combat ranks. Don't forget, SHIFT applies the and
operator to the selection, meaning all units rank 1-3 get selected, and rank 0 is the special invisible rank that gets filtered out of most selection actions.