Yes, only the rules changed with the 77 edition.
From the Wiki on the game's home page;
D-Day (second edition) Wiki wrote:
The original version is termed D-Day 1961A. The rules were revised in later 1961 or early 1962 (version 1961B). The mapboard and rules were revised in 1965 by Larry Pinsky and the rules were again revised in 1977 by Jim Stahler. The box and counters have remained constant. D-DAY was the first of the "classic" games which remains popular in postal circles to this day (1980), although the 1977 edition rules are a vast improvement and have breathed life into a game which was previously fast becoming forgotten.
And, as you can see, it also gets lots of play-by-mail action.