Is it correct to say that dissolving i a packet of juice powder in water makes a new substance, fruit punch, and so it must not be a physical change?
No. Although it looks different, it is not a new compound altogether. It is simply dissolving powder into water, allowing waters of hydration to surround it.
This "new" fruit punch is simply hydrated juice powder, which is not a new chemical compound, but an anhydrate plus water molecules surrounding it.