Official rule text
23.4 UNITS: Units are limited to those included in the game, except for fighters and ground forces. a When a player would place a unit, if there are none of that type left in their reinforcements, that player can remove a unit from any system that does not contain one of their command tokens and place that unit in their reinforcements. A player can remove any number of their units in this way; however, any units that are removed must be placed immediately. Abilities cannot force a player to remove and place a unit in this manner. b When producing a fighter or infantry unit, a player can use a fighter or infantry token, as appropriate, from the supply instead of a plastic piece. These tokens must be accompanied by at least one plastic piece of the same type; players can swap infantry and fighter tokens for plastic pieces at any time.