There is only a vacancy on the EC if the owners at the last AGM agreed that the EC must have a certain number of members (this is an obligatory item on the agenda) and there were not enough nominations, or if one member resigned or their unit was sold.
If there are vacancies, all an owner has to do is nominate someone to take the vacancy – if there are more nominations than seats, then there is a ballot by EC members. Otherwise, valid nominees can just take the vacant seat.
If there are no vacancies and you want to change the EC, you need 25 percent of owners to sign a letter calling for an Extraordinary General Meeting and then put forward a motion to vacate all the seats on the EC and elect a new one. Obviously you wouldn’t do that unless you had people prepared to serve on the EC.