Even though the current war in Ukraine has led to a short-time renaissance of coal-fired power plants, the age of coal-based power generation is about to end. Nevertheless, the coal-fired power plants no longer needed for their original purpose are still valuable assets. In this paper, we consider three technological approaches for repurposing them: gas-to-power (operation of combined cycle gas turbine plants), power-to-gas (operation of electrolysis plants for feeding hydrogen into the gas grid), and a combination of the two technologies mentioned above. Our aim is to find optimal operating modes in terms of profit for the three approaches. For this, we use a mixed-integer linear multi-variable optimization model and time-resolved price forecasts for electricity and gas for 2030 and 2040. Our results show that, also in future energy systems, gas-to-power plants allow for economic benefits: In times of district heat demand, they operate in the spot market and profit from dual revenues. Balance and ancillary markets allow for additional revenues from the capacity provision. Power-to-gas plants do not show the same good economic performance. However, they allow for an economically sound operation and gain most of their profits in the spot market. Compared to the others, combined plants do not offer economic advantages. In our paper, we also investigate the currently high energy price situation. It allows for payback periods of power-to-gas plants as anticipated for 2040. For this reason, long-term high prices may accelerate the deployment of such future technologies.