If you make fairly minor design changes, then you would be liable for infringing on their intellectual property (e.g. trademark or copyright) or unfair competition; potentially, you would have to pay all profits to the company whose design you took, possibly pay additional compensation, and also likely have to stop selling the knock off.At some point, the changes would be so great that it's a new design; unfortunately, there is no hard and fact rule as to when that happens. For example, say that company A has a best-selling sweater design which has a pine tree pattenr on it. Clearly, using that design but simply adding christmas ornaments to the tree would be infringement; on the other hand, changing colors, changing the pine trees to palm trees, and also changing either the size or placement of the trees would be a new design that simply inspired by--not derived from--company A's shirt. Somewhere in-between those two extremes is the line between infringement and not.