I received this email as well. What's interesting to me is that the sender's email address is do_not_reply@new.itunes.com. Is that even a legit Apple email address? I'd expect to see @apple.com if the email were coming from Apple.
And while I now know that the AOL conversion by March 31, 2015 is real, I do take issue with another factor in emails like this. I was addressed with a "Dear iTunes Customer" line. That immediately caused a red flag to go up in my mind. As a rule, I trash any messages of this nature if they don't address me by my real name. And Apple DEFINITELY has my real name! So if the email is truly legit, why doesn't Apple use my real name?
I'm positive that the email is a phishing scam. Why? Because it tells me to visit support.apple.com/kb/HT204268, but the code under that hyperlink actually would take me to a page under the new.itunes.com site. So if this email is legit, why would they show a legitimate support article address, but have a user's click take them to a completely different host site and page?
In summary, and I'm sorry I made this so long, I think everyone should use caution. The text in the email is absolutely true. Users who have used AOL screen names to access Apple services DO need to update their accounts! But this email coming from new.itunes.com, not addressing me by my real name and then including a cloaked link (looks legit, but leads to a completely different destination than expected) seems VERY fishy (or phishy? -yeah...I had to go there and be punny) to me.
I hope this helps someone out there avoid having their account compromised!