User blog comment:Linathan/Unit Spotlight: Chrono Mage Elaina/@comment-71.232.207.27-20150503171553/@comment-26140304-20150503223116

That's true. If anything, Gumi could release a different batch of units that require different versions of the evo stones. Another possibility is that they could make evo stones materials for future 7* units that they may release, similar to Bulbs.