How can I modify a trigger at subscribers SQL Server 2005? I have a SQL Server 2005 publisher with SQL Server 2005 subscribers and one SQL Server 2000 subscriber running merge replication. 

Is there any way to modify triggers at a subscriber with the replicating schema changes set to false (replicate_ddl = 0) without reinitializing the whole publication? 

A solution where the triggers are only modified on the SQL Server 2005 databases is fine.
Yes, you can use the system stored procedure sp_addscriptexec to run an ad-hoc script on all the subscribers.