Yes, Ultimate Park Hopper passes are good forever. We always buy a 7 day Ultimate Hopper Pass and use 4 days one trip and 3 the next, or if 3 isn't enough, we'll upgrade to another 7 day UH pass. We find it to be the most economical way for us, without locking ourselves into the annual pass.
I've been in September too, and it is a great time to go, few crowds, value season, warmer weather than January so it's great if you want to do a water park.
Have a great time!