Provide a detailed threat assessment of current RSA-2048 and Elliptic Curve Cryptography (ECC) standards in a post-quantum landscape where Shor's algorithm is efficiently executable. Compare lattice-based cryptography, hash-based signatures, and multivariate polynomial cryptography as viable replacements. Outline a migration roadmap for a global financial institution that prioritizes backward compatibility without compromising long-term data confidentiality against 'harvest now, decrypt later' attacks.