They could migrate accounts automatically, I'm sure.
To me, it seems that Bungie are trying to get as many Reach players as possible to link their Gamertags to Bungie.net accounts.
They've also asked us to link our accounts to emails.
Why?
Because I think that they're going to be migrating the userbase of Bungie.net into the Aerospace system.
They'll take the usernames of all of our accounts here on Bungie.net, set up accounts for us automatically, send temporary passwords to our email addresses (which will act as our log-in address too), and then we will be given the option to change our password once on the site and set up.
We'd then be able to connect our Gamertags, PSN accounts, Apple Accounts, Facebook, Twitter, etc. You name it.
There's no way that they'd sift through members manually.