I saw this on another website, but I can't remember where -- I thought it was a neat way to get rid of a texture you hate: "If the texture is not too deep, level and smooth the walls using drywall compound. It would be very difficult to REMOVE the texture... and some wall damage could occur... so using drywall compound is the better and less difficult way to go. Use the first coat to fill in all the major depressions. A 12" taping knife is essential for leveling broad surfaces. The second coat refines the first coat. Use the taping knife... dry... to knock off any high spots from the first coat. Then fill in the defects left in the first coat. After the second coat dries, you might want to do a little sanding with a 120 grit paper or sanding screen. A vacuum with a sanding screen attachment is the lowest dust option. Now apply the third coat of compound. This third coat may not even be necessary, or at worst should be a final leveling of any remaining unevenness in the wall. Do a final sanding with a 220 grit paper or screen to get the walls flat. Vacuum thoroughly and wipe the walls with a slightly damp sponge... rinse frequently... to remove any residual dust and smooth in any scratches from the sanding. Prime the walls with any high quality primer intended for drywall. If any stains or discoloration have appeared in the compound, use a stain killing primer such as Kilz. Then apply sizing over the primer before wallpapering. There are special paint removers (try a paint store) designed to remove heavy wall textures, also, though I can't testify to their effectiveness from first-hand experience. If the texture is water based, you can also use a wallpaper steamer to soften it for scraping but you might damage your walls."

Scraping won't do it. You'll damage the paper face of the drywall, and even if you didn't, you'd just end up with bare drywall that would still need finishing. The real reason builders texture drywalled walls is to cover flaws in the wall surface (uneven studs, extremely rough "rough framing", etc). If you look at a drywalled wall before it's been textured, you'd see it's got all kinds of ripples and bulges.

As described in another post, it's pretty easy to apply and sand drywall topping. But it takes a lot of practice to develop the skill to float a smooth, straight wall. If you want a straight, smooth, plaster-wall finish, you're probably going to need a professional. But if you'd be happy with a slightly uneven finish, you could definitely do it yourself. With a not-so-good DIY job, dips and ripples will be plainly visible in any light. With a good DIY job, the imperfections will only be visible with a light shining across the surface (do you have canned lights around the perimeter of the room that wash the walls? this will highlight any defects). With a good professional job, the walls will appear straight and smooth under any light.

Be aware that the dust from drywall topping is EXTREMELY messy. It's a fine dust that floats and will get all over your house. Work one room at a time. Empty the room where possible, cover floors and anything you can't remove with drop clothes, and hang a drop cloth across any doorways to keep the dust from spreading through the whole house.

Since you'll be "plastering" the entire walls, you'll be generating a tremendous amount of dust. You'll need a shop-vac with a drywall dust filter (drywall dust will clog the regular filter in just a few minutes). To minimize the dust in the air, you can get a drywall sander that attaches directly to the shop-vac. Another option is a sanding rig that puts a bucket of water between the sander and the vacuum. The air from the sander is bubbled through the water before going to the vacuum, reducing the dust (and wear) the vacuum has to deal with. I haven't tested it myself, but if I was doing a big job like you're looking at, I think I'd try it.