Yes, but you can't take back old licenses from anyone who got it before you changed it (unless they specifically say you can, which no FOSS license does, by definition).
But anyway, I think this is about code that's a derivative of someone else's and so not entirely your own.
No, the idea here is that after all the original GPL licensed code (not own code) was step by step fully replaced by by own code (Ship of Theseus). Then you can change the license, because it is 100% your own.
There is plenty of legal case law on this ranging from BSD to various proprietary licenses. You would not win on this. This is why Compaq had to do a clean room implementation of the BIOS back in the 80s -- the law of doing it otherwise was already clear back then.
But anyway, I think this is about code that's a derivative of someone else's and so not entirely your own.