Ph.D. Topic Problem- Changing Topics/Labs
Whichever makes you a happier person. Life is short and not worth spending 5 years in a condition you don't like. If you have the money, go for it. If not, here is my longer answer.

The work experience you are getting in field A should not be a priority because you are there to do research, to contribute to science. Work experience in that field should be just a tool so that you can earn money and live properly. The excellency of people in field A are also irrelevant, as your research is in field B.

But, life does not always present us the best options. If you cannot get a scholarship when you switch to field B, then I would definitely stay at where you are now. You say you can cover expenses, but if you don't get a scholarship, how much will you lose in 5 years? That's a big number and imo not worth it (unless you have a million in some account)

I would stay there and from time to time, consult professors that work in field B. After some time, as you dig deeper, you will know more than those guys anyway.

It really depends on your budget I guess. I am not rich so I don't like the idea of Ph.D. level scientists' paying for it.