A simple logistic model of market substitution has been applied to situations where price and cost parameters vary in time. In such cases the market share does not necessarily evolve as a monotonous function of time. Although this particular feature of the model has not been validated against data from past experience, it can be applied heuristically to studying future trends in market penetration. As an example, possible competition patterns of coal and nuclear power as primary fuels for electricity production have been examined under various combinations of economical parameters. The illustrative results involve both cases where the nuclear contribution would be phased out as well as those where nuclear power could still maintain a strong position a few more decades to come.