As you found, the cabin filter is pretty easy to change.
When I first got my LS, I was confused about the cabin filter/ac filter and the Seat filters. There are air filters for the front seats. Maybe there is some confusion regarding these.
The seat filters are really more like screens. There is one under each front seat. The hardest parft if getting your hand under then and pulling it off. They are about the size of the palm of your hand. Mine looked clean at 50,000 miles, but an air gun would do the trick.
BTW, I am cheap. I clean my cabin filter with an air gun. Paper filters are easily cleaned this way.
Also follow the instructions on resetting the computer message exactly. It is a little tricky to reset the filter message the first time.
Good Luck,
Jim Henderson