It's not chemically different, they just pastuerize it at higher temp for shorter length of time to ultra-pastuerize the product, and they add the lactose stuff too, I guess.
I used to buy lactaid only because it lasted so much longer than regular milk, but lately I've been buying goat milk which also has a long shelf life. I get it at my local chain supermarket, although I haven't found it all that many places . It is so much richer than any cow's milk, and is great to cook with. I am totally hooked on it.
Of course Lactaid milk is chemically different. And of course you know you can't have goat's milk. Lactaid adds the enzyme you are your loved one lacks--lactate--to the milk to break down the milk sugars your body can't. This is why it is curdling at a lower temperature. The digestion process starts in the container. Below is a the manufacture's website and they welcome questions such as yours. Click on the Ask The Expert's box and good luck.