Aged coal-fired thermal power plants (TPPs) still have a large share in electricity production. In Europe average age of coal-fired TPPs is about 37 years, while in US it is 44. These plants are of a great concern today due to carbon emission and their lower efficiency and flexibility. There is a general commitment to shut down these plants, but that can’t be done overnight due to the rising need for electricity and the large investments in utilization of intermittent renewable energy and in inevitable energy storage. Therefore, coal-fired TPPs will play their roll in a transitional period towards net zero emission energy systems. Consequently, it is important to upgrade efficiency of aged TPPs in order to reduce emissions, besides achieving better economy. In addition, upgraded flexibility would support power control in electric systems with increasing share of intermittent solar and wind renewables in power generation. Hence, improved balance-of-plant designs with moderate investment costs are needed with the aim to upgrade efficiency and flexibility of aged TPPs. In recent years a number of solutions for the increase of efficiency of aged TPPs were investigated, manly with an advanced utilization of flue gas waste heat at the boiler cold end. The upgrade of TPPs flexibility is suggested by installation of additional heat storage equipment or by utilization of available energy stored in working fluid on the expense of reduce efficiency. A review of these methods is presented, while a detailed results are provided for two solutions conceived and investigated at the University of Belgrade: (a) an additional economizer installation with innovative connection to the main feedwater pipeline, which was built at the 650 MWe lignite-fired TPP and which provides both energy efficiency and flexibility upgrade, and (b) a steam accumulator installation in the steam turbine plant of the TPP with the aim of thermal energy storage and TPP’s flexibility upgrade. These new designs enable both primary and secondary power control with no changes of firing rate and fresh steam temperature, pressure and flow rate. In case of the steam accumulator application, there is also no change of the process parameters in the feedwater and condensate lines, while the application of the additional economizer leads to redistribution of the feedwater flow between the main and additional economizer and only to a slight feedwater temperature change at the boiler inlet. These features reduce TPP’s aging, which is otherwise more intensive in cases with standard methods based on the change of firing and accompanied with the change of fresh steam parameters. The prediction of TPP operation with additional economizer is validated by comparison of calculated process parameters with available operational data measured during TPP steady-state and transients. Also, the steam accumulator behaviour is validated by comparison with measured pressure transients. The application of new designs for efficiency and flexibility upgrade is justified with demonstrated environmental, economic and operational benefits.