Developing both Applied and Theoretical Interests
Short answer is yes you can. Now, how/why do researchers change directions? Many reasons; 


Chasing different funding/proposals venues.
Being part of a school or department where interaction of professors and available lab equipment can lead (with time) to you redirecting your research.
When advising students for MS/PhD or being a part of search/research committees, one tend to think outside the box (outside of his research area) and incorporate different methods/ideas
You learn more new stuff with time! Remember that a PhD in subject X does not mean that you get to work in X for the rest of your life. It simply means that you know X (in details) much more than other people (or professors).