Is it possible (read that easy) to change a very late 3d generation Lyman 48 WH from Stayset knobs to Target knobs? Specifically is it a drop-in fit?

Brownells did have the spare knobs for some models (elevation)

It's possible but not necessarily easy. I've done it, it usually requires the swapping of the entire screw assembly rather than just the knobs. You'll need some small trustworthy screwdrivers and probably some non-marring clamping device. I swapped the parts from a target-knob sight with the parts from a stayset-knob sight and everything worked OK.
Regards, Joe

Yes that was what I suspected. The fellow I got it from luckily has one with target knobs to swap.

Brownell's only sells knobs for Lyman 57 and not 48 sights. Thanks to all.