Space agency officials decided to gamble and replace a discredited design in the shuttle booster's nozzle assembly with an alternative design tested successfully last summer. At the same time, they said, engineers will continue analyzing the design that failed in a test-firing on Dec. 23. Engineers have determined that half of the nozzle's eight-foot-diameter boot ring popped off after the two-minute ground test was completed, said J. R. Thompson, director of the Marshall Space Flight Center in Alabama. The problem would not have affected an actual launching, officials said. The solid-fuel booster rocket made by Morton Thiokol Inc. has undergone redesign since the shuttle Challenger exploded on Jan. 28, 1986. Meanwhile, in response to a published report about a "second flaw" found in the December test, NASA said a small hole piercing a nozzle-joint sealant was "not particularly harmful" and proved that the new joint design did its job.