Abstract: This research is aimed at resolution of the question of forecasting tax revenues for consolidated budget of the constituent entity of the Russian Federation in terms of changing the composition of taxes within taxation system under the conditions of high level of uncertainty. For assessing the economic effect from transition of small business entities from Unfixed Tax on Imputed Income (UTII) to other special tax regimes, the author analyzes the established by legislation conditions, requirements and limitations applicable to special tax regimes, and formulates the benchmark data for forecasting. Based on the results of analysis of existing approaches towards tax planning, it is substantiated that the forecast assessment of tax revenue after cancellation of UTII requires employment of expert approach with the use of the method of scenario analysis. The scenario leans on the fact that the business interest of taxpayer, which defines his choice in terms of changing the taxation regime, is the reduction of tax burden. The key variables for the scenario analysis include: legislatively established restrictions towards application of special tax regimes by the taxpayers, as well as the amount, categories, and organizational-legal forms of small business entities that after the cancellation of UTII selected another taxation regime. The result of approbation of the proposed approach consists in the forecast assessment of economic regime of transition from UTII to other taxation regimes, reflected in change in the volume of tax revenue tor the consolidated budget of the Republic of Crimea. The proposed methodology is valuable for the simplicity of calculations and accessibility of the necessary database, and can be utilized for forecasting the tax revenue of any constituent entity of the Russian Federation in the context of changing taxation of small businesses.