So, the problem in a nutshell is I need to be able to keep track of when subscribers change their email addresses and tie them all back to the single subscriber.
Here's what I did - I set the Subscriber Key value to be a numeric identifier. I then created a data extension and I set ID as the Subscriber Key. I imported a sample data file with ID of 62078213 tied to email@example.com. I sent myself an email and then clicked Profile Center, and changed my email address.
Here's what happened. Before I made my change, I checked _Subscribers and found my record that was newly added from the email send. Awesome. Okay, then I updated my email address to firstname.lastname@example.org from the stock preference center. I then checked _Subscribers again:
select * from Cxxxxxxx._Subscribers with (nolock) where ID = 62078213
Running this query brought back the UPDATED record in _Subscribers - which had email address as the updated email@example.com email address. Further, I did
select * from Cxxxxxxxx._Subscribers with (nolock) where EmailAddress = 'firstname.lastname@example.org'
Recall, this was the ORIGINAL email address that was sent the email.
No results. So, I don't know where the "historical" email addresses are stored, or if they even are - several people I've talked to mentioned that this is the way to go about it if you want to tie a subscriber back to multiple email addresses. I wonder if there's a business rule that needs to be turned on. Any insight would be terrific, if you know of anything I'm missing.