A method of controlling the polymer composition of an ethylene copolymer in a process for preparing ethylene copolymers by copolymerizing ethylene and at least one other olefin in the presence of a polymerization catalyst system comprising at least one late transition metal catalyst component (A), at least one Ziegler catalyst component (B), and at least one activating compound (C) by adding an alkyl alkoxy silane or a dialkyl ether for increasing the relative portion of the ethylene copolymer component obtained from polymerization by late transition metal catalyst component (A), or by adding a saturated halogenated hydrocarbon for increasing the relative portion of the ethylene copolymer component obtained from polymerization by Ziegler catalyst component (B), processes for copolymerizing ethylene and at least one other olefin in the presence of such a polymerization catalyst system comprising utilizing the controlling method, a method for altering the polymer composition of an ethylene copolymer obtained by copolymerizing ethylene and at least one other olefin in the presence of such a polymerization catalyst system and a method for transitioning from one ethylene copolymer grade to another by using the method for altering the polymer composition.