Not only can, but if the instructions change then must.But somebody told me that you can change the instructions after you mod or add a design to the blank shirt. Cause the washing instryctions change
The instructions need to be accurate. That doesn't just mean washing it as per the instructions won't harm the garment, it means the instructions need to be as non-restrictive as possible.
So saying "do not bleach" on a garment that can be bleached isn't allowed.
So averaging all your shirts to have one set of instructions if they should have more than one set of instructions would technically be a violation of the FTC's rules. A small one, but a violation all the same.