I looked around Google and found this video about changing the BB on a Brompton.
It seems old models use ISO square tapered BB while new models use JIS square tapered BB. So whichever new crank you're going to use should match the same standard as the BB. The guy in the video used a new JIS BB but an old ISO crank, which seems wrong. The tapers are at different angles, so the crank won't fit right with the spindle.
If you plan to switch to cranks with integrated spindles, this blog site documented such a modification. You have to be careful with the chainline and frame clearance, as described in the blog. One thing the person didn't seem to mention is shaving the water sleeve for the Shimano MTB BB to fit in between the MTB cups. I did a similar modification for my Sugino cranks on my Dahon bike because I needed to move my chainline further to the right. Since the Shimano MTB BB cups are 1mm thinner than the road BB cups I could shim the right cup further out while conserving a little bit of spindle length. I shaved a road water sleeve to fit the MTB BB cups, but I guess you could also shave the MTB water sleeve too if you shave both sides.
https://handsonbike.blogspot.com/201...nkset-and.html