Changing research interests as an undergrad?
Your undergrad career is actually the best time to explore different research interests. It is much easier now, when you are an undergrad, than it would be at any later stage.

There is no ethical problem with changing your research interests after working for a lab over the summer - working for a lab as an undergrad is absolutely not a commitment to stay in that lab for a PhD.

Similarly, you are absolutely allowed to have research interests that are different from your undergrad dissertation. Many, many students change subfields in between their undergrad (or MS) and PhD.