I entered current school as a Master-Ph.D. integrated course in high energy physics, however, decided to graduate as a master in this year and will apply abroad in the next year since my research interest is different from that of current supervisor. The defense is upcoming, indeed is on this Friday.
I'm considering changing MS thesis advisor because I'm uncertain but I guess my current supervisor may write a bad letter of recommendation. I had several troubles with him mainly because his research field and style is different from mine. He implicitly mentioned that I took other student's chance of being his student because my interest was different from the beginning. I asked him explicitly if he can write a letter for me, and although he said he will write the paper, many of what he said after this implied bad contents. For example, I asked him if I regularly report what I'm doing in next year, whether he can re-evaluate me or not. He politely rejected this saying once I graduate from his lab, I and his life should be independent and he shall not care about mine. He also mentioned that evaluation should arise naturally, not in such an artificial way.
My defense is on December first, so it's within this week. But I have been seriously considering to changing supervisor and to postpone graduation to the next semester. I already got approval from a faculty to whom I want to transfer. He also explained the project I will take on if I change the supervisor to him.
However, I am very worried that it would be very rude and inappropriate behavior to change supervisor due to the worries about a bad letter and within such a short interval before the defense. I'm also worried that doing this will make current advisor as an enemy. But I am confused because he had suggested me to change the supervisor at the end of March this year.
If I graduate this semester and get a bad recommendation letter next year, even though I got strong ones from other faculties and researchers, will it be a significant weakness in the Ph.D. application?
Can any advice or opinion? Thanks.
P.S. In a comment I left, I mentioned that my advisor rejected advising me. To be specific, he told me that he is very busy so he will not advise me in person but instead he will ask two PDs in the lab for my thesis. He seemed very busy, but I was first student of his the only two students, and I know that he is regularly advising and being in discussion with an undergraduate student for his graduation thesis. Also, he explicitly mentioned that since he doesn't trust my ability, he can't allow extending my work beyond the elementary scope when I asked him for adding contents beyond the Standard Model on my thesis.
P.S.2. I am quite certain that I can get a good reference from the faculty to whom I want to transfer. Even though I graduate this semester, I'm planning to work with him next year. In this case, if I have a bad reference from the past supervisor while also have a strong one from another faculty with whom I will work next year, will it seriously affect my application?