I have over the years looked into doing a PhD in a different area from mine. I was particularly struck by this paper by Belnap on causal origination and found that a Dutch philosophy group had received funding to look further into it.
There was a PhD place going and the stipend was enough for both of us to live off in Holland. It turned out that the funding protocol forbade anyone who already had a PhD. I applied for a related Postdoc but was told that they wanted a trained philosopher and not a scientist. I had figured that I could probably attend philosophy lectures and do catch up.
Similarly in the UK I started looking into doing a PhD in a Fusion at one of the doctoral training centres. An ex-academic seeking to start over was weird enough, but the funding rules were the same. If you already have a good enough bachelors to apply you can get funding provided that you do not have a PhD.
There is a funding paradox, if you are already qualified to do research in one area you are not able to get funded to do research training in another.