The jump between modification and redefinition is very big in my opinion. In some ways I think it should be, but there really is nothing much going on between substitution to modification (ie. I don’t see “significant task redesign” in the modification part).
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SAMR Image source: Dr Ruben R. Puentedura http://www.hippasus.com/rrpweblog/ CC-BY-NC-SA 3.0