The car selection process concludes when all drivers within the league have a car and teammate assigned.
Once the first race has taken place, no car changes will be allowed at any point during the season. Intentionally using the wrong car in an attempt to benefit yourself or others in a race will see you disqualified from the results of that race.
If a new driver joins the league at a later stage of the season, they may pick any car that is available at the point of being assigned to the league.
You will not be able to switch, sorry to inform you of this.