You can tear down your plaster if you want too and replace the old plaster with drywall which will make the home look nicer but you don't have too.
Be aware that it's a big and messy job so lay down tarps on your floor or something else to protect the floor from the plaster and plaster dust.
You'll also have some lath boards behind the plaster which is what the plaster is stuck too so you'll need to remove those lath boards as well.
If the walls are exterior walls where you remove the plaster and lath then that's a good time to add insulation to those exterior walls to keep your home warmer.
I prefer to have a drywall finished house as it's also easier to hang things on the wall and makes the home look nicer.
Replacing your plaster with drywall can increase the resale value of your home because it looks nicer and won't start crumbling like plaster would.
I would replace the plaster on the ceilings with drywall too so that you don't have to worry about plaster falling off the ceiling which it will eventually.