Graduate Research After Retirement
This is harder to answer because you seem to want to change fields. Otherwise, and if you are near to a research university, you could just ask to be allowed to attend research seminars in the field you already know. You would probably be welcome and could give some help to various students with their own research. 

However, to change fields, there are things you need to know to get to the research frontier in the new field. I assume you know this, of course. The ideal way, assuming that money is no issue, would be to apply for another degree in the new field. If it is close enough to your current interests it would probably be expedited to some extent. 

But the way to get good advice is to visit some place that you might be interested in and talk to researchers there. I'm guessing that this would be easier with a university than elsewhere. Even better if you already have some research ideas. But note that the request is a bit unusual and many places don't have the time or inclinations to do things that don't fit a given mold. 

Top research universities differ quite a lot in what is possible. Some might be extremely rigid about rules and requirements and others might be extremely flexible. So you may need to look around a bit. 

There is one additional issue, however, that might get in your way. If anyone starts to think that by working for "free" you are displacing someone else who might be early in their career, you won't be terrifically welcome. You will need to finesse that.