My memory may be incorrect, but I seem to recall that the costumer who came up with the original stormtrooper designs had a side business selling copies to fans and cosplay folks, was sued for some sort of IP infringement, but eventually prevailed.
My theory on the redesign is that Disney wanted to be certain they unquestionably owned all the rights, so they had a new version designed and all properly registered well before release of the newer movies.
