Abstract: A process for transitioning between Ziegler Natta (Z/N) polymerization catalysts and Second Polymerization catalysts in a polymerization system which includes at least two polymerization reactors. During the transition, the Z/N catalyst is used in the “downstream” reactor, thereby providing a flow of polymer to finishing operations. This eliminates the need for lengthy shutdowns of finishing equipment and thereby improves the efficiency of the plant operations.