You're really not going to do that cost effectively. How or what would you "fill" in the area with? DIY or are you hiring a mason?

Honestly, I don't see them as ugly.

I was going to say what GardenGal said. If you think they are ugly, they are not as ugly as water stained on the interior of your walls.

These are very effective, functional brick sills which are designed and installed to prevent water from entering your house, and causing mold and deterioration inside your wall framing.

Do you really want to remove all windows, hammer out these brick units, replace them with other sill pieces (what would you use...matching brick from your exteriors, cut to fit and slope away from the windows, or ???), reflash and replace windows, and repair damage to and repaint the interior wall surfaces? Really?

Have your received any estimates on the cost of this?

Masonry window trim is a style factor usually found in very expensive homes. What do they look like after pressure washing?

The only functional deficit of brick run-offs vs. stone is that the mortar between the bricks may have to be redone every 30-50 years.

Natural stone windowsill.

evening it out with the rest of the wall?

Windowsills can be decorative, but their function is to keep water out of the house.

I have them too. Mine look just like the picture that worthy just posted above me. They are wonderful. Why would you want to remove them. They do have a function, and they do that function very well. They keep water out of your house and are certainly not ugly. They are on used on brick or stone houses.

Even the groove under the sill is there for a reason. It stops water running back under the sill and down your wall. I have stone sills with just such a groove. They've done their job for two hundred years.

I agree with what everyone else is saying that these sills serve a purpose and you do not want to remove them. But if you do not like the look of them maybe you could try to paint them to give your house a trimmed look. Or you could put a thin layer of stucco over them to make them look like 1 solid piece. If you did stucco them you'd need to be very careful about how you apply it

I had deep windows like these at my last house - my house was built of natural limestone with very thick walls and windowsills. My outdoor cats loved them! They could jump up and easily sit on the sills, watching me working in the kitchen.

Removing these window sills is not "updating" the house. It's a wasted effort. They're fine. Paint them the color of the trim. If the trim is the same color as the house, pick a good trim color.