The main change I know of is they lowered the lift and changed the duration to get more torque at lower rpms. A no-longer available letter to dealers and service managers that used to be on their Heritage pages explained the design change and said it was done to help reduce smog in bumper-to-bumper traffic (I suspect mostly for LA and NYC). Some time ago I printed a copy of it but the last time I looked couldn't find where I put it and as I said after the pages were changed it along with some other stuff disappeared. As I recall they said they made the changes to improve fuel mixing and combustion at lower rpms. What I have here shows in 1958 the lift (for solids) was intake .4050 and exhaust .4141. In 1959 for solids was .3105 intake and .3325 exhaust. I think the change in torque and rpm was more pronounced with hydraulic lifters (which by then were all you got in passenger cars). Makes sense to me as passenger cars probably would have contributed to the smog problem more.

Parts of this make sense to me when I look at the Carter YF3211S and it might answer why Carter never redesigned the YF for the 235 after 1957. It has a 2 stage rod and the same size main jet as the YF2100s. However the economy size of the rod is .004 larger than the 2100 (meaning it will run leaner) and the power size of the rod is .002 smaller (meaning it will run richer). If the changes to the camshaft produced more power at lower rpms (which happened) it seems they'd cause the 235 to make less power at higher rpms and maybe the idea was to have it run richer in the power setting to offset that. Just guessing on that but the camshaft did change.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end