You want to replace all the drywall in the house? Absurd! Would take you weeks, unless you are a pro.

Removing the popcorn is a snap if it is the standard soft stuff. Use water and a putty knife. I give you all the details on my website at http://www.bobf.info.

To change the wall orange peel, you could try applying the knockdown over what you have to see if you like that. If not, try sanding it down some and then applying the knockdown.

Applying knockdown would be a long process for you if not experienced. It takes a pro longer than orange peel by a bunch and that is why they charge extra. Read the drywall topic on my website where I tell you how to texture.