No, it cannot, because M Aggron EX and Aggron ex are different types of cards. One is an EX, and the other is an ex. Blame the Japanese R&D team for the confusing presentation, since, as far as they're concerned, the fact that one is all caps and the other is lowercase and italicized means they're completely different card types. Unlike in English, lowercase and uppercase Latin letters are not interchangeable when used within Japanese. Even though EX and ex largely function the same (they're both cards that are dramatically more powerful than their peer cards that give two prize cards when taken down), EX cards are exclusively Basic Pokemon unless they're Mega evolution cards (denoted with the M), while the older ex cards included a lot of Stage 1 and Stage 2 cards amongst their ranks.
Now if you had an Aggron EX, it could evolve into M Aggron EX. You just can't get to M Aggron EX from Aggron ex.
