pt-online-schema-change tool will work for existing triggers for a table?
According to the documentation, the tool accomplishes its magic of keeping the old and new tables in sync while the copy process is happening via the creation of its own triggers and is therefore not compatible with tables already using triggers no matter what kind of change you intend to make.

Any modifications to data in the original tables during the copy will be reflected in the new table, because the tool creates triggers on the original table to update the corresponding rows in the new table. The use of triggers means that the tool will not work if any triggers are already defined on the table.
— http://www.percona.com/doc/percona-toolkit/2.1/pt-online-schema-change.html