No serious migration project can be deployed without a plan to make it incremental
No amount of testing will ever guarantee perfection. Parts of the migration can and will go wrong for a variety of reasons, and you cannot count on luck alone for your project to succeed. You must survive the bumps that will occur. You must be able to phase your project, and have the ability to revert each of these phases if anything happens.
More than likely, your management will not take the risk of a migration project guaranteed by testing alone. And if they do accept this risk, the blame for any mishap will be on you. You were the one who claimed that testing would do.