The array is only reallocated when it runs out of space. There is no magic about the "old" array. The GC just won't collect it, as long as a pointer to it exists.

I don't think there are many reasons, if any at all, to keep a pointer to an array element of a slice around in Go. Usually I only get the address of an array element only when passing it to some C code or doing some low level manipulation, but then I don't keep the pointer around. In Go code you usually just keep the slice object - which contains the necessary pointer anyway.