I am an incoming physics student at a UK university for a 1-year master's degree. I just completed my undergraduate studies at a US university and deferred a year for their PhD admission (currently planning to come back next year).
However, the master's program and PhD program have a completely different focus: the PhD program is in the same area as what I have been working with my undergrad advisor for 2 years. If I come back next year I will likely continue working with him in this area. The master's program, on the other hand, focuses on a different branch of physics in which I don't have previous experience, but I really want to learn something new to me.
A few months ago I posted this question, where I asked about the possibility to decline my deferred PhD offer and then reapply. However, I'm not sure if this plan is mature enough. I want to explore more about the new area that I'm about to study and then I think I will have a better idea of what I want to do for my PhD.
My concern is I want to be respectful to my current advisor and the department. I know in principle I probably should come back next year as I have asked (and been approved) for the deferral, but I still want to keep my options open, in case I really find the new area fascinating with me and change my mind. I don't know if it sounds convincing to talk to my current advisor I want to reapply to the new area in which I don't have enough expertise (compared to the current area I've worked for 2 years with him). I'm also thinking about spending a gap year after the master's program and then making the decision. In either case, I will need his support to write me a reference letter.
What's the most respectful way I can do to let my current advisor know I might have a different research interest, and ask for his advice?