No sealant needed. And you really don't need to drain it all out of the block, you can if you really want to.

I've always just replaced the upper and lower hoses at the same time as a coolant change, that way you've got new coolant and new hoses.

The plastic petcock on the lower tank of the radiator has a rubber o-ring on it. All you'll need to do is open it and open the top of the rad. and drian all the coolant. Keep away from pets (toxic), take it to a parts place and they will recycle it.

Then fill back up and you should be good to go. I like a 50/50 in the winter and a 70 water/30 coolant in the summer.

Joe