On all the existing levels, you can toggle easy / hard before entering the level (select the level, then before starting the battle, click on hard up above the monster preview), provided you have done the easy once.
Season 6 / 2B / Legends of Atlantis requires that you put the whole level in “hard mode” to select the hard versions, so the selection comes before doing the level, and as a result, the hard levels must be done in order — no deciding to kill the end boss early.
Yeah I’ve noticed that when I switch over into season 2 where the Atlantis levels are, each one I click on that I’ve already beaten on normal, gives me the option to do normal or hard. However on part one, even though I have cleared pretty much almost all of it every time I click on one of the levels, it does show me the mob preview, but there is no option to select between normal or hard like there is on part 2