will it still delete mine enen if there is open save slots
It doesn't "delete" your game so much as starts your right back at the very beginning but now in Hero Mode. You get to keep all of your bugs and materials, but none of your weapons or items.
-There are NO heart drops AT ALL in hero mode until you find the Heart Medal again, which isn't until Lanayru Desert, so the first bit of the game is quite challenging. Even with the Heart Medal, the Heart drops are still pretty rare in dungeons. In all of the breakable pots that used to contain hearts, there's now nothing. Makes for harder boss battles.
-EVERYTHING causes double damage, including when you run out of air underwater, or when you touch lava or spikey things. Enemies that take one heart off per hit now take TWO. Enemies that used to take two hearts off per hit now take FOUR!
-You get to keep all of your bugs and materials, so upgrading is pretty easy. I was able to max out my upgrades each time I found something new to upgrade, so that's pretty awesome.
-Some characters acknowledge you are in Hero Mode and will give you the option to skip certain things (the snoring Kikiwi at the top of the Great Tree recognizes you and if you select the right answer, will let you skip his cryptic clue about Farore's symbol).
-You can skip almost all cutscenes and establishing shots
The only thing about Hero Mode that I feel like they could have done better with is made it so you could skip things like Loftwing training and took Fi's constant instructions out. If they know we've already been through the game, they shouldn't have Fi telling us every trivial tiny thing again.