[addaon]

> Unfortunately, there is no easy way of changing B with permanent magnets.

There’s the old semi-permanent magnet trick, although this only allows a couple of discrete field strengths, not continuous tuning. Basically, a neodymium (or other hard) magnet and alnico (or other soft) magnet are put in parallel, surrounded by a pulse winding. A pulse can flip the magnetization of the alnico, but not neodymium, so you get field strengths of either neo + alnico or neo - alnico. (This is most often used sized for similar strengths to turn a “permanent” magnet on or off, but can be used in non-equal strengths as desired.)