Shinui Koneh - this one requires a bit of background

The Rosh in Perek 9 of Baba Kama Siman 3 asks the following question. If one steals a pregnant cow and it gives birth, must the gazlan pay the value of the cow as it was stolen, or must be pay it's value a moment before birth. The Rosh says at first that you probably must pay the value a moment before birth because the increase in value belongs to the nigzal. He compares the case to one who steals a barrel of wine that appreciates in value. The halacha is that if you break that barrel you pay the higher price because it appreciated in the "possession" of the nigzal.
In the end, however, the Rosh concludes that the gazlan pays like the time of the gezeilah, because that is the general rule, and there is no similarity to breaking the barrel because there he is destroying the object and it is like he is becoming a gazlan at that moment.
This Rosh is very strange because he never makes the obvious distinction in comparison between when the barrel breaks on its own accord, or when the gazlan breaks it. In the case of the cow that should be the determining factor, what caused the shinui? Yet, that factor never appears in the Rosh at all. R' Chaim learns from this that only when the item is completely destroyed do we say that one is considered a gazlan when he breaks it. However, if all that happened was a shinui of some type, so there is no difference at all between the item changing of its own accord or not.
Nevertheless, it is difficult why the Rosh never mentions this distinction at all.
To answer this question we must analyze why when the item breaks on its own the gazlan is not responsible as a gazlan.
The Rosh in Merubah Siman 2 explains that the reason is because a gazlan never is mekabel shmirah.
The Shitah Mekubetzes in Baba Metziah 43a says that in fact a gazlan is a shomer. Rather, for whatever reason, only when the gazlan actively breaks the item is he considered a gazlan again at that moment.
Acc. to this we can explain that the Rosh holds when the item breaks on its own, forsure there is no mechayev at that time, for a gazlan is not a shomer. The only question is by shinui. There the shinui koneh may be mechayev the gazlan. This doesn't exist where the item is destroyed because there is no shinuy koneh. This explains the hava amina and maskana of the Rosh.
In the Shitah Mekubetzes however, a gazlan is a shomer and still is not chayav if the item breaks of its own accord. In that case the same would surely be true by a shinui, that the gazlan would not be chayav at the time of the shinui if it was a shinui memailah, because it is entirely comparable to a case where it broke of its own accord.