The key to understanding the Sugyos of Shinui in Bava Kamma and Sukkah
Michtavei Torah records the following letter from the Rogatchover:
Note the last two lines!
Rabbi Menachem Kasher in the Mefa'aneich Tzefunos elaborates:
A couple of sites that explain chomer and tzurah are:
(first of four posts)
Now, we can understand all types of shinu'im very well: The yeiush of the original owner only applies to the chomer of the stolen property. The shinui is necessary to eliminate the original tzurah of the object so that too can now be considered to be under the aegis of the ganav or gazlan.
תן לחכם ויחכם עוד
ישמע חכם ויוסיף לקח