Cahnging research topic after the mail

I emailed the PI about the topic I am interested in, and their repsonse was positive, they said that this topic is related to the project, is meaningful and timely and that they look forward to see my research proposal.

However, after doing the literature review I found another topic which is also related to the project but different from the initial topic I sent the email about. The new topic is also related to the research project but different to the proposed topic which i wrote in the mail tho its wayyyy coooleer.

Will it be lame to make a research proposal on the new one coz its really wayyy coool and bettter then the initial one.

PLZ HELP TTTT