I also just became a Bronze PowerSeller and it happened automatically. I was quite surprised to see this because I have just been selling stuff from around the house. I will happily take a 5% disount in fees though
I do not know if the higher levels are still opt-in or not.
I am pretty sure they are automatic also. I know for a fact the silver is.
I sold a few lots of scrap gold by weight and that kicked me into the silver power seller status automatically for 3 months. After that it was back to normal at bronze.