Substitutions for canned diced tomatoes
If the soup recipe is eventually going to be pureed, then yes, you can substitute with little problem. 

If the soup is supposed to be a little chunky, the substitution will change the texture a little, but the flavour will probably remain the same, and all in all it should be fine.

Note that this assumes a can of pureed tomatoes. If what you have is tomato concentrate (it'll be marked as 22 or 28 Bx usually) then you can't substitute directly, as you'd need to dilute.