You do realize sockaddr_in is an abstraction for data structure here, yes?

https://datatracker.ietf.org/doc/html/rfc791#page-11

Where is that new address going in the header?

If it's going in the same spot in the packet header as the current IPv4 address, how do you make sure that the 20-30 routers owned by 3 different companies that are likely to be between your computer and the destination computer exhibit a behavior that is consistent with moving packet closer to the destination?

(If they don't, you've just made a version of IPv6 that is worse-- it's missing the last 30 years of IPv6 implementation.)