After the completion of The Boston Major regional qualifiers, Valve introduced changes to the substitution rules in Major tours. From now on, any player, who is not a member of the main roster of any team-participants, can substitute another player.
The update was not announced by Valve corporations. The changes appeared in the FAQ on the team registration page.
This way, any of the 16 teams to participate in The Boston Major can take any player for substitution. This has a huge impact on the teams: though the official substitutions are prohibited, a substitute can join the team if a player from the main roster has problems with visa or some other problems. The player from the main roster should confirm the participation of the substitute. Valve need to take additional measures to prevent the cheating attempts.
Currently, there are no clear rules on substitutes. Previously, a team could use a substitute in very exceptional circumstances of force majeure when all the substitutes were mentioned in the roster of the team. How a new rule will affect the event, we will see soon.