Different rules mean a different game in a more fundamental way than more developed body types mean a different game.
Here are some sample rules changes from the early era of baseball.Note that major rule changes--and there were many more than listed here-- were made almost annually in the period between 1865 and 1895. They were literally making up the game as they went along.
My source is
http://www.baseball-almanac.com/rulechng.shtml1867 Pitcher's box is a 6 foot square. Pitcher may move around inside the box.
1879 The number of called balls became 9.
1880 Base on balls reduced to 8 "called balls"
1883 Six "called balls" became a base on balls
1884 All restrictions on the delivery of a pitcher were removed.
1885 Chest protectors worn by catchers and umpires came into use.
1887 Calling for high & low pitches was abolished
1889 Four balls became a walk
1891 Substitutions were permitted at any point in the game
1891 Large padded mitts were allowed for catchers.
1893 Pitching distance increased from 50 feet to 60 feet 6 inches
1893 Pitching box was eliminated and a rubber slab was substituted
1893 The pitcher was required to place his rear foot against the slab
Now, to my way of thinking, 1893 is when we begin to have a sport that one can think of in the same terms as modern baseball. A game in which there are 9 or 8 or 6 balls for a walk isn't much like today's game. Neither is a game where the ball is thrown so softly that catchers don't need chest protectors or padded mitts.
And a game where the pitcher stands in a large box 50 feet from home plate soft-tossing called high or low pitches to a catcher unprotected by a real mitt is fundamentally different from a game where a pitcher is standing 60' 6" away with his back foot on a slab, firing fastballs over 90 mph to a catcher who can defend his vital regions with a padded mitt and chest protector.
Important rule changes continued on a fairly regular basis until 1920, after which rule changes became rare, but the pace of change slowed after 1895, and for my money it's 1893 when we start to get something like real baseball.