I still think Jimmy Page did in fact borrow that opening chord change from the Spirit song, simply because he did the same thing with so many other songs, however I also think he and Plant added enough of their own material to make it a new beast altogether, and so the estate of Randy California doesn’t need to get anything for it (in fact before his death Randy himself said he didn’t care that they stole that little bit from his song.)
This is very different than the issue with Jake Holmes’ “Dazed and Confused”, Anne Bredon’s “Babe I’m Gonna Leave You” or Willie Dixon’s “You Need Love” to which Zep added parts and improved upon, but are still clearly the same song with a fancier arrangement.
