How difficult to switch from manual testing to automation testing
It depends. Test automation is a different field, you will have learn new skills and have a paradigm shift of how you look at building and testing software. So if you have a hard time adapting to a new world this might also be a challenge. For quick learners and progressive people this might be easy.
Also you need programming skills. If you find learning programming hard then the transition will be hard. There are record and playback tools, this might make it easier at first, but any test automation expert is a well skilled programmer.