In the Lynda Carter's Wonder Woman TV series, or in the old comics set during the World War 2, did Steve Trevor ever come to know that Diana Prince is Wonder Woman ?
I don't recall Steve Trevor learning Diana's true identity in either Golden Age comics, or on the show. I wouldn't be surprised if there was some story where he learned of her identity only to have that knowledge gone at the end of the issue, or what were referred to as "imaginary stories" (yes, I know, they all are), explicitly set outside the normal continuity where he new her identity.
However, in Wonder Woman (vol. 1) 223 (released in April 1976 [probable cover date August]), Steve is brought back to life (he'd died while Diana was a non-powered adventurer), and (due to having seen stuff while dead) he knows her identity. Just a few issues later, the comic switched from telling present-day tales of the Earth-1 Wonder Woman to telling WWII tales of the Earth-2 Wonder Woman for a couple of years. When they switched back to Earth-1 again, Steve managed to get killed off again almost immediately.
A "permanent" change to the status quo - if not one that was fully explored, or long-lived.
Not quite what you requested, but the closest I can come.