What do you think of alterations being done to an artists work after that artist's death?

It seems that books are continuously revised to appeal to the current world. Old English becomes new English. Is this really right? The same is done with music. Old music is changed to appeal to a new audience/ to make money. I guess we do it to all things including paintings etc. Someone will paint the Mona Lisa with a moustache or grin on her face. Sometimes the new version smothers the original. Like processed food which has so many additives art has becomed 'processed' too. Any thoughts on this.