On a Merkel, I'd opt for no 2 - change front/rear both. Pity that Merkel doesn't offer decent sights, but they're far from alone. I find square notch/post sights useless on a rifle.

A double is really at it's best with express sights. The problem with express sights today is that few of those on the new guns are any good. The rear should be a wide, shallow V, with the face sloped forward and a platinum center wire. The front should be a flat faced bead (I prefer platinum) that's not too large. I don't like anything over .080". Beads any larger than that are no easier to pick up quickly, and just cover more minutes of angle to no benefit.

Also, as well as having a flat face, the bead itself should lie flat on the face of the stem, and not be enlongated like the one in your second photograph. Enlongated beads like that one give me fits from glare and sidelights.

No, done professionally, such work should not require a re-black.