The Incredible Hulk Vol. 1 (1962-1999) #4

Plot

Rick uses some of Banner's machinery to transform the Hulk, who was stuck in his monster persona, back into Bruce; Bruce is incredibly weakened by the transformation and takes a chance in using the machinery on himself again which transforms him back into the Hulk's body but with Bruce Banner's mind; Now Bruce and the Hulk can switch at will using the gamma ray machine.