A theoretical framework is built for capturing properties of competition in mature monopolistic digital product markets. Based on an empirical study of the market of accounting software for small and medium enterprises, a consumer choice model is suggested, where a rational consumer is already using a particular version of a software package and is considering to chose from the following three options: either to continue using it, or to upgrade to a newer version of the product, or to switch to a competitive product. Consumer decision is justified by software quality, and network effects, under the price and switching costs constrains. A modified consumer demand function is used for the model, and theoretical conditions are analysed for choosing from one of the three above-mentioned options. The results are applicable to a wide range of digital products.<br /