My parents are green card holder, they are here in US right now. They got their green card last year. I want to know how long can they stay out of US, without having any problem with green card. I heard it is 1 year. What do they need to do if they want to stay more than 1 year? They have their address of MA but now have moved to TX, do they need to fill AR 11 form or simply can change their address from Post office?