The coming into existence of the statue is said to be a substantial change. It is not a change in the statue itself (note: I question this), but rather a change in what substances (substantial particulars) exist. Substantial change occurs whenever a substance begins or ceases to exist.
One way for this to happen is for the parts of something to be rearranged (this is how the statue comes into existence). Another way is for some things to become united together for the first time (this is how a house or a record collection comes into existence).
It is possible that: for some substances x and y and sortals f and g: x is the same f as y but x is not the same g as y.
Necessarily: for all substances x and y and sortals f and g: if x is the same f as y, then x is the same g as y.
(A relation R is sometimes said to be non-symmetric just in case it is neither symmetric nor asymmetric. So it is possible that there are x and y such that x stands in R to y, and it is possible that there are x and y such that x does not stand in R to y. Note that being non-symmetric is different from being not symmetric.)
Necessarily: for all x and y: x constitutes y at time t iff x and y exactly coincide at t, and every part of x at t is a part of y at t, but not every part of y at t is a part of x at t. (Does this assume that both x and y are complex - i.e. have parts?)
It follows from this account that constitution is asymmetric - thought to be a point in favour of the account.
Anyone who thinks that the statue is not identical with the lump of bronze but is constituted by it, will probably also think that the head of the statue is not identical with the part of the lump with which it coincides, but is constituted by it.
Tibbles is a cat. Tib is that part of Tibbles which is everything but Tail, Tibbles' tail. Tibbles is not identical to Tib, because Tibbles has Tail as a part but Tib does not. Suppose that Tibbles loses Tail (which it can do, and still be Tibbles). Tib still exists. But now Tibbles and Tib exactly coincide.