Full Idea
There is 'alteration' when the substratum is perceptible and persists, but changes in its own properties. ...But when nothing perceptible persists in its identity as a substratum, and the thing changes as a whole, it is coming-to-be of a substance.
Gist of Idea
If a perceptible substratum persists, it is 'alteration'; coming-to-be is a complete change
Source
Aristotle (Coming-to-be and Passing-away (Gen/Corr) [c.335 BCE], 319b11-17)
Book Reference
Aristotle: 'The Basic Works of Aristotle', ed/tr. McKeon,Richard [Modern Library Classics 2001], p.484
A Reaction
[compressed] Note that a substratum can be perceptible - it isn't just some hidden mystical I-know-not-what (as Locke calls it). This whole text is a wonderful source on the subject of physical change. Note too the reliance on what is perceptible.