Var.: Волк каждый год линяет, а всё сер бывает
Cf: The fox changes his skin but keeps his knavery (Br.). The fox changes his skin but not his habits (Am.). The fox may grow gray (grey), but never good (Am., Br.). A leopard cannot change (never changes) his spots (Am., Br.). The wolf can lose his teeth, but never his nature (Br.). The wolf changes his coat (skin), but not his disposition (nature) (Br.). Wolves may lose their teeth, but they never lose their nature (Am.)Русско-английский словарь пословиц и поговорок. Академик.ру. 2013.