This actually appears to be a legit Apple emailing. I have family members who have received the same and have been researching it for them.
I believe this goes waaaay way back to earlier days of the iTunes rollout when, if memory serves, an authentic AOL Username could be used as an Apple ID. Since then, it has become common (required?) that an Apple ID is an authentic "email address". The difference between a Username and an email address being the "@aol.com" designator.
It appears that Apple (and AOL) are requiring anyone that has or had an iTunes account using an AOL "Username only" Apple ID (without the @aol.com email designator) to update their Apple ID. I believe it to be as simple as designating an authentic "email address" as your new Apple ID.
If you already use an email address as your Apple ID, i.e. user-name@aol.com, then I believe there is nothing further you need to do.
If I am incorrect here, I'm sure someone will speak up and let us know.
I hope this is helpful.
d.i.