All I know about that is I tried to migrate my Mojang account to Microsoft - when Microsoft started "unifying logins" - and I still haven't heard back from them. So my experience has been negative. Hopefully they don't try to migrate Battle.net accounts in the future.
FYI you have 2 months to migrate your account before they delete it. You will have to repurchase the game on a new account if you ever want to play it again if you don't migrate it by September 19th.