Is it legal to alter RN documentation?

The HHA that i work for have a ''Quality Assurance'' department that reviews all things OASIS. We use a HI-TECH computer device and the info gets downloaded directly to the QA department who reviews and makes changes as neccessary. If any changes are made, they send us a message on our device letting us know of the changes made and reasons for the changes, we are also able to call them to discuss changes. I think that having the QA department is the reason for this company's great success in the area, they are rated number 1 in the are according to Medicare. I'm not sure of the legalities but so far it has proven to be non-problematic.