Is it OK to change my research tools/methods completely after/at the end of my PhD?
Undoubtedly, to me, the answer is yes. I am free to learn whatever I want, whenever I want, and do the research I want, as long as it is following right scientific methods. I may even think of multidisciplinary research where I can learn other disciplines and integrate them in my research. Science is all about curiosity and continuous learning and exploring.