As usual something that sounds as simple as changing ring size can be complicated. Besides the minor shimano issue or non-issue of A-set versus B-set, it turns out that shimano only made a 42T middle chain ring for 9-speed.
It's possible that an early ultegra 10-speed (6603) middle 39T ring might bolt up and have workable spacing, would also require a matching front der.
There's also a current cheap shimano 9-speed (2200, below sora) 39T that I couldn't bring myself to buy.