From the upper deck behind third base, where you are almost high enough to touch a midday cumulus as it floats by, the sweep of the baseball field below is as large as the 162-game season about to begin. It is opening day in April, but across the breadth of spring, summer and autumn is a season, filled with sweets and sours, long enough to satisfy all baseball tastes.

At Memorial Stadium, the Baltimore Orioles were playing the Kansas City Royals. All seats were sold, all hearts taken. The players came in from Florida spring training the day before, their bodies stuffed with vitamin C from citrus and their minds drilled with the fundamentals that win more games than home runs.

For 107 years we have been a baseball nation. In tactics, the game has been near changeless. The run-scoring strategies of John McGraw in 1908 are those of Tom Lasorda today. When we take our children to a game, we are showing them what our grandparents saw.

But off the field it is different. There, baseball is being changed in ways that look strange on paper and stranger on the tongue. The game may soon become more than its players.

The "sabermetricians" come first. That's what members of the Society for American Baseball Research (SABR) are called. Through their computers, they are providing every chip of statistical data that a manager can use to win games. With a dugout computer, a manager won't flash signals to the bullpen coach to get a relief pitcher ready. He will punch his Apple II for a printout to learn which left-handed reliever throws the fastest slider over the upper right-hand corner of the plate to a right-handed pull-hitting batter, and who is up next.

Sport magazine, in its current issue, believes that computerball is about to lead to "a small revolution." The changes "will be subtle, perhaps invisible to all but the most devout fans: Players will be positioned in the field more precisely, strategies will be determined by mathematical probabilities and not by old-time baseball lore."

One team going up to the plate with a hard-hitting sabermetrician is the Chicago White Sox. Last year, Sport reports, the Sox used a computer "for pitch-by- pitch analysis. At the end of the season, they used their findings to redesign the contours of their stadium, deciding to move the plate closer to the fences when they discovered that of the 83 fly balls that were hit to the warning track last season, the Sox hit over 50 of them."

The other day, the art of information retrieval didn't help the Chicagoans in their season opener. They lost 5-3 to the Texas Rangers. The first baseman, apparently not consulting the printouts before taking the field, bobbled two grounders in the first inning. The errors led to two Texas runs.

Baseball has always attracted data fanatics. Until now, though, they have been fans in the seats, not managers in the dugouts. Over the years--I go back to Ebbets Field and the Polo Grounds --I have noticed that the cheap seats attracted the most extravagantly informed statistics nuts. They spent their money on baseball statistics books. They read ancient box scores as though divining the Talmud.

In Baltimore, the management appears intent on converting everyone to the joy of stats. A player comes to the plate and the scoreboard lights up with some arresting skinny on him. When Ken Singleton took his turn with a man on second base, we were told that in 1982 he hit .342 with runners in scoring position.

I go to the games to see baseballs batted around, not numbers. A hitter with the full count on him still whips up more tension than a scoreboard telling his full history. It's the apple in his throat we should be watching, not the Apple II in the dugout.

After the Orioles-Royals game, I happened to pass a basement used- book store a few blocks from the stadium. There on a back shelf, collecting dust as old as John McGraw, was a Ring Lardner book. It was a treasure second only to coming home with a foul ball. In the book were some of Lardner's "lost" baseball stories, including one written in Colliers in 1912.

The game he wrote about 70 years ago is the same we watch today. But there is a difference: off the field is about to become off the wall.