I thought this would simply be about the knife and the fork switching hands, but holding the fork tines up or down (spearing vs scooping) is new to me.

On the other hand, I don't think Americans ever pick up food with their fork and switch the loaded fork to the other hand, especially if the food is scooped, not speared. A lot of food would be dropped in the process.

As a non-conformist, I taught myself to use my knife in the non-dominant hand so that the fork is used in the same hand regardless of knife usage.