Unfortunately, Max, "legal" is not the issue. If the change would impact your required rest or days off or maximum flight time under the AFTLs, you are perfectly safe in refusing, politely, the new duty. However, if you are going to try to justify not accepting a change of a G day to a work day just because the rostering practices say it can't be done, you'll be jeopardizing your job. That's how it is at Cathay. You can show them something written in black and white, they'll call it gray and say, "So, what are you going to do, sue us?" Yep, that's how it is... Sorry.