Of course, you can change your regular glasses to sunglasses. Just take your brown frame eyeglasses to any optical store and tell them that you want to change your eyeglasses to sunglasses. Generally speaking, your eyeglasses can be added some tinted colors to the lenses and then they just look like sunglasses. Choose any color you like to tint your eyeglasses lenses to make them like sunglasses. If you want them have UV protection, you should ask them to add UV protection coating to the lenses so that they can protect your eyes from harmful UV rays.
Yes, you can add tint on your brown frame glasses lenses. That will give you a pair of new tinted sunglasses. Now, most of glasses shop support that tinted sunglasses. Besides, you can also add UV coating if you add tints for your eyeglasses lenses for the sake of your eye health. The tinted lenses can help reduce strong light when you in strong sunshine while the UV coating can prevent UV rays when you expose to sunshine.