ne way to do it:

Dump trigger definitions into a file

# mysqldump -uroot -p --triggers --add-drop-trigger --no-create-info \
      --no-data --no-create-db --skip-opt test > /tmp/triggers.sql

Open triggers.sql file in your favorite editor and use Find and Replace feature to change DEFINERs. Save updated file.

Recreate triggers from the file

 # mysql -u USER -p DATABASENAME < /tmp/triggers.sql