IPv6 was conceived with the idea of coexistence with IPv4 for a large period... That is why, there is no "migration" date, or D day, but only a transition mechanism that allows both protocols to coexist. There are, for this purpose, more than 40 transition mechanisms proposed at the IETF. This is only a matter of choosing the more convenient to our needs.
The field ‘Total Lengh’which is part of the IPv4 header, is not found in the IPv6 header. Its function was to count the size of the packet payload plus the size of a variable lenght header. As the IPv6 header has a fixed size, the presence of this field is unnecessary.