Bicycle Mechanics - Help! Differences between 9 and 10 speed Cranks!

hi- can someone settle this once and for all for me? it is my understanding that the only difference between 9 and 10 speed cranks is the thickness of the chainrings. 10 speed has thinner ones that help it to shift easier. however, as i begin looking for a 10 speed crankset, i see that 10 speed cranks are advertised as specific, as if there is a difference with the actual cranks, besides price. Can anyone tell me if this is true, or are they simply just different because of the thickness of the chainrings?
My reason in asking this is if i keep my old cranks, can i just add new chainrings, or do i need a new crankset? thanks so much in advance!

no difference.

You shouldn't have to change anything. Use your crank just the way it is.

9 and 10 speed cranks are the same. Many people associate outboard bearing styles with 10 sped since it was then that they first appeared, but you can mix and match will no ill effect.

9 speed cranks will space the chain rings slightly farther apart. If you run a 10-speed chain on a 9-speed crank, the chain MIGHT get sucked into the gap.

The chain rings by themselves are the same (ie same thickness, tooth-gap, bolt-hole spacing)