How to do safely ALTER TABLE for changes in column types with pt-online-change-schema

pt-online-schema-change will do alter tables without affecting at all the production table locks. It runs completely silent and is pretty safe. 

 If we want to check what exactly the pt-online-schema-change will do you can first do a dry run which will output what exactly will run 

 pt-online-schema-change --alter "MODIFY COLUMN xmlSent MEDIUMTEXT" --dry-run --print --alter-foreign-keys-method=auto D=emplemdb,t=novusXMLTrans -u root -p"!....!" 

 ΠΡΟΣΟΧΗ: ΝΑ ΓΙΝΕΤΑΙ ΕΛΕΓΧΟΣ ΟΤΙ ΥΠΑΡΧΕΙ ΧΩΡΟΣ ΣΤΟ ΣΕΡΒΕΡ. ΑΝ ΕΧΕΙΣ ΛΙΓΟ ΧΩΡΟ ΕΧΕΙΣ ΠΡΟΒΛΗΜΑ 

 To actually run it later on  

 pt-online-schema-change --alter "MODIFY COLUMN xmlSent MEDIUMTEXT" --execute --print --alter-foreign-keys-method=auto --progress percentage,1  D=emplemdb,t=novusXMLTrans -u root -p"!....!"