Official Team Manual (OTM) “Tournament Information and Rules”:

Players Must Maintain Current Membership

If your membership is not current, you are not eligible to participate in the World Championships. If any of your teammates are not available at renewal time, consider renewing for them so they do not get dropped from the roster and become ineligible. Notify your Local League Management immediately if a player is accidentally dropped from the roster.

Qualified Teams Must Maintain Active Status

A team that qualified in the Summer Session must play during the following Fall and Spring Sessions in the League area in which they qualified. A team that qualified in the Fall Session must play during the subsequent Spring Session in the League area in which they qualified. Local Bylaws may impose additional requirements.

Spring Rosters Are World Qualifier Rosters

Your roster at the end of the Spring Session will be your team's roster for the World Qualifier. Players who were on the roster when the team qualified but are not on the roster in the Spring Session leading into the World Qualifier, are not eligible because they are no longer considered part of the team. THERE ARE NO EXCEPTIONS TO THIS RULE.

A team with only four available or eligible players on their roster will have to play in the World Qualifier with those four players. All roster changes for qualified teams must be made prior to the teams submitting their week four scoresheets during the Spring Session. All qualified team rosters are frozen at the conclusion of week four. No one may be dropped or added. THERE ARE NO EXCEPTIONS TO THIS RULE.

Teams Must Retain Four Original Members

In order for a team to maintain its qualification, the team's World Championship roster must include four original members. Original members are those who were on a team when it gained qualification to a World Qualifier.

• A team that qualifies in the Summer Session must retain four original members in the subsequent Fall and Spring Sessions.

• A team that qualifies in the Fall Session must retain four original members in the subsequent Spring Session.

• Any player on the Spring Session roster, who was on the team when it first qualified, is considered original.

• An original player who does not meet the minimum matches played requirement prior to Playoffs will be ineligible but will still be considered original.

All above rules apply to both the World Qualifiers (Cities) and World Pool Championships (Vegas).

Please review all details around this topic in the OTM.

Other references:

Bylaws 

Website