A method is provided which reduces the transition time and/or the polymer waste in a continuous polymerization plant and/or process having a back-mixing reactor when polymer grades are changed from a first polymer grade to a second polymer grade. A monomer(s) and processing agent(s) are introduced to the reactor. The concentration of the processing agent(s) in a feed stream to the reactor is varied as a function of time from a first value associated with the first polymer grade to a final value associated with the second polymer grade. During the concentration variation one or more intermediate values of processing agent concentration are adjusted between at a first value, an intermediate value(s), and a final value. The intermediate values may be maintained for a time which is calculated on the basis of only residence time and steady-state correlations between input and output of the reactor and/or of the polymerization plant. The method is performed without performing dynamic modelling.