Purple gear doesn’t change into epic gear, but epic gear can be turned into legendary gear by using crimson spinel. This is why people spend long amounts of time in the level 60 dungeon, to try and obtain all of the parts of level 60 epic equipment that they need. You typically get epic equipment directly from dungeons, not by changing anything else into it.
You don’t really need legendary gear until you reach level 60. The level 60 legendary gear is what you need to obtain level 70 and ult. level gear, by upgrading it with crimson spinel.