Moreover, a Biclique attack against AES exists, by saving some meet-in-the-middle computations, it has already reduced the full 10 rounds, 128-bit AES to "just" 126-bit (25% of 128-bit) of security. Is it a clever attack? Yes. Does it mean the security of AES has been reduced to 25% of the original security level? No. Does it practically matter? No. This is exactly why 128-bit security is seen as a minimum standard in cryptography - it can provide a more-than-adequate security margin which renders all minor speedups in cryptanalysis irrelevant.

If the 64-bit random serial number has already provided an adequate security margin, it should be that no action needed for all existing 63-bit certificates. But it seems the choice of 64-bit here is arbitrary without good justification...