Thanks, Sheldon. I go by your word and knowledge.

I understand that the chain must become narrower with the change from 8-speeds to 10-speeds, as the cogs must also get closer to each other to acomodate a larger number of them in the same space.

But I'm not sure in what way are 10-speed shifters different from 8-speed shifters, within Shimano.
Is it the total lenght of the cable pulled as one shifts from the smaller cog to the larger that's different, or is it the same lenght, but that lenght is divided in 10 increments instead of 8 increments?

Obviously, the reason I asked is because new shifters cost a motherload compared to the rest of the drivetrain.