I'm sure the federal statute states that a mag is illegal if it is ALTERED to accept more than 10. Therefore, if you did change the follower, whether it had any effect or not, the mag could be said to be altered. If it is a stock M10 mag, I don't believe that there is any law against using a .40 mag in a 9. I've read that poeple do this with USPs, and I even read on sigforum that a guy had all 3 barrels for a 229; 9, 40 and 357SIG. He claimed to have fed 40s reliably from a 9 mag.
If you live in a non-free state it's probably illegal to carry more than 10, but in a free state, I don't see a problem.
pojim