I'm not too versed in auto transmissions, but you've made two changes at once so it's hard to say which one is the problem. I'd be surprised if just changing the fluid was causing a problem, because that would mean that the new fluid formulation would make your otherwise properly functioning transmission slip. So either the new fluid is much lower friction, or your old fluid was really worn out and was hiding the fact that you actually need some transmission work. Neither of those seem especially likely, so I don't think it's the fluid.

I'm more inclined to think that the valve spring change has had an effect on something else, even if it is just a little trapped air somewhere. It is ultimately a hydraulic system, so all the same problems that brakes and hydraulic clutches have also apply here too.

That was my way of saying I don't really know. LOL.

I don't think 100mL of fluid either way would cause a problem with anything. That's only 1/10 of a liter, not very much for a trans that probably holds multiple quarts of fluid. There should be no problems with anything if you're in the right dipstick range.