Chaning Themes

Once I install WordPress, and select a theme (which I am having a hard time with, but that is probably another forum) and start adding content, how hard is it to change themes? For example, I select something very simple and basic, such as SIMPLR and I have been blogging for a few months. Then, I decide I want a different them, say, Talian… Would it be difficult to change themes… AND maintain my previous content?

Ed