How do I combine transactions for two accounts after the BOA migration?

You can just this process to move transactions from one account to another.  In this case I would remove any duplicate transactions in the new account, including the opening balance transaction and then move all the transactions from the old account to the new one.
One warning, this will not transfer attachments.  You could try with "Move Transactions" instead of "Copy Transactions", but because of a long-standing bug in Quicken it would prompt on every reconciled transaction.  You could workaround that by changing the reconcile state to say "c" before the move and change them back afterwards (it will still prompt on transfers if the other side is reconciled).
FAQ: How Do I Move Transactions Between Quicken Accounts? — Quicken