Finally. (And yes, this a true finally). A couple hours ago I saw this lovely morsel from Apple Employee #8, Chris Espinosa:
I ended up with two different Apple IDs ages ago. All my iTunes purchases were on one account, and at some point I had done a .Mac (which later became MobileMe, and then iCloud) trial, where I had registered a second Apple ID.
I had bought into MobileMe when it first came out, so I could have the convenience of cloud sync, but the email address I wanted was on my old .Mac trial account. So I resurrected that, and had then gotten myself into the odd reality of needing to use the .Mac/MobileMe/iCloud account as my primary account, while all of my purchases were on the other account which Apple thankfully supported, but always felt flimsy.
The predicament became even flimsier when I got married, and my wife and I decided to share my secondary account for purchases so we could both use the music and apps we bought, because we got married before Apple's Family Sharing existed.
When our son came of age to really have his own devices, we made the jump to Family Sharing, and thankfully somehow this Primary/Seconday account setup still worked. But it has always made me feel uneasy. Heck, about a decade ago I even emailed Tim Cook about the need for a way to consolidate this stuff, and got a call from his (likely one of many) personal assistants to get more details about the request.
I'll be waiting a little bit to see how this migration tool fares with other folks. I'm too risk averse to be one of the first guinea pigs. But I do plan to do it and finally get all of my iCloud data and purchases under one roof.
Cheers to everyone at Apple who made this tool happen!