I noticed that the base sharpness setting did seem to change in some firmware versions, there were a couple of versions around 3.11.0 (can't remember exactly) where the sharpness was lower, then later it was increased again. The base weight might have changed a bit in some versions too.
If you wanted to get it exactly the same as it was in a particular older firmware version then you'd need to adjust the weight/sharpness settings to match by eye, as they changed the base sharpness not the default setting. I.e. setting readingFontSharpness\Georgia=0.10 won't necessarily give the same actual sharpness in all firmware versions. 0.10 in one version might be equal to 0.15 in another version.
Unfortunately the separate sharpness slider disappeared after version 3.1.1 so for later versions you'll need to adjust the sharpness by editing the configuration file.
Edit: You could use the weight/sharpness demonstration kepub in
this post to help make the adjustments. If you take a screenshot with it set how you like it on your old firmware, then another screenshot in the new firmware, you should be able to work out by comparing the screenshots side by side how much you need to adjust the sharpness to get it to match.