As with anything made from fabric, corsets will stretch out a little over time and after a certain amount of wear.
What is important to realize is that this is not a sign of poor quality and simply a fact of physics.
Much like a brand new pair of jeans will have a degree of rigidity to them and we wear them a few times to "break them in", so it is with a corset. But rather than referring to it as "breaking in" your corset, we call this seasoning your corset. This process allows the corset to gradually mold to your unique shape and curve smoothly to your enhanced silhouette.
Also, rest assured that what we're talking about here isn't that your corset will stretch out to the point where you will find yourself considering a size or 2 down within a week of wear, no no. Any good quality steel bone corset will feature a twill lining and that lining will be fused to the out shell of the corset for additional strength.

So to re-cap: your corset will experience a bit of give or "stretch" after a bit of wear but nothing to the degree where its shape will be altered or warped. Rather this will give it the softness required for it to both follow and enhance the shape of your natural curves and silhouette.
Happy lacing!