I am a PhD student in computer science. My current advisor was also my masters advisor, and I stayed with him for a PhD because back at the time the research I did under him was interesting for me. Two years into the PhD, after talking with friends from grad school and exploring different fields in CS, I found myself increasingly drawn into a different topic. I waited for a while, just to make sure this is not a "whim", but a real change in my interests. At this point I am absolutely sure that new field is what I want to do (and also in terms of after the PhD).
My current advisor cannot advise me in this new field, since this is not his field. I talked to the dean of my department, and he said that in this case it makes sense to find a new advisor, and just move. I am ok with starting over after two years, but I really feel bad leaving my advisor, after they spent so much time and effort in me. They funded me for the whole time, helped me navigate through the university bureaucracy, helped me to get scholarships (the last one is a pretty big one which I just received, and it made me feel so bad to accept it and leave. Needless to say, when I applied for it I did not know that leaving my advisor would be the option suggested by the dean).
I do not have one sharp question, but I need advice: how common is it? Should I offer to return the scholarships? How should I present it to my current advisor? (they don't know about me planning to leave, but it won't be a surprise for them at this point)