REVERSES OREGON TRAILPORTLAND, Ore., April 1— (AF)—-A trip back over the old Oregon Trail, on horseback, was started today.The Rev. ..John W. Beard, 65, retired Presbyterian minister and chaplain in both World Wars, and Mrs. Beard, began, the long trip to Independence, Mo., over the route which peopled the West and led to organization of the Oregon Territory 100 years ago.Accompanied by a son, he traveled the Columbia river by canoe from its source in Canada to its mouth, and later followed the Lewis and Clark trail down the Missouri river.They plan to cover about 25 miles a clay.