( saying ) (you cannot) successfully make people change their ideas, methods of work, etc., when they have had them for a long time
(YOU CAN'T) TEACH AN OLD DOG NEW TRICKS
Meaning of (YOU CAN'T) TEACH AN OLD DOG NEW TRICKS in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005