pt-online-schema-change trigger problem
As you may know, pt-online-schema-change works in this way:


Creates a new table identical to the original one, but empty
Alters the new table
Creates triggers on the original table to start keeping the tables in sync (update/delete of rows already copied, insert of new rows)
Copies the rows in bunches
Swap the table names


So triggers are a fundamental part of the process (unless the table is readonly or you are definitely sure that no one is going to modify it now). The problem is that, before MySQL 5.8 and MariaDB 10.3, you can have only one triggers for each combination of table + timing + event. For example, your table can have only one trigger BEFORE UPDATE.

One thing you can try is, turn your existing BEFORE UPDATE triggers to AFTER UPDATE - unless you already have an AFTER UPDATE trigger, this will do the trick. And do the same with INSERT and DELETE triggers.

Or try to use gh-ost from GitHub, as someone already suggested. This would work because this tool doesn't create any triggers: instead, it reads the binary log and applies the changes to the new table manually.