Depending on the degree

These illustrations are from a classic german medical publication "Neue Heil" and compare the trained and untrained skeleton on the left, as well as the altered organ position, shown on the right.

Note that the organs have moved, but only to a modest degree compared to   pregnancy, or... just when we move around!

Tightlacing tends to inhibit

vigorous activity or exercise. Then, it does makes sense to loosen the corset or to remove it, also to avoid soiling it with perspiration. Someone who is usually very active physically may want to chose a suitable corset style, like a ribbon type corset that will still preserve the small waist, but with less lower chest constriction. However, if your goal is to significantly narrow your ribcage, as produced by a wasp waist corset, it is better to trade off particular activities. The other reason for the adopting upper thoracic breathing, is the pressure against the midriff from the abdominal content below. Tightlacing corsets usually extend longer below the waist to prevent undesirable abdominal bulging. This is turn increases the upward  pressure against the diaphragm and thus further inhibits abdominal breathing, but by allowing time to adjust, the overall effect is very tolerable especially for women. Compared to men, women are naturally predisposed to breath less with the abdomen with or without a corset on, and aside from that, it is not very graceful to watch a rising and falling stomach with every breath. This effect also exists during pregnancy where the growing fetus displaces the abdominal organs and significantly increases the intra abdominal pressure leading to shortness of breath and heartburn. Again there are parallels with pregnancy, where abdominal breathing becomes inhibited as well, and where constipation and heath burn can occur.

Much has been written about starting corseting at a young age, yet this was primarily intended to reduce the overall training effort. By starting before or at the onset of puberty, much less effort is needed to form the ribs and upper body. The consequence of beginning corseting from an early (10-13) age, is the promise of a very slender adult ribcage but it also allowed for an easier long term training process. For the modern wearer, who starts at a later age, the very small waist dimensions can still be achieved, but it will take a lot more effort and time, however the very narrow ribcage shape will be more difficult to attain, but still some late starters have been successful.  - see accomplished modern tightlacers.

Lastly, tightlacing of the lower chest reduces stomach capacity, giving a sensation of a full stomach after only a few bites. This can be a helpful tool in a weight reduction program, but only if worn sufficiently tight for 23/7. The moment the corset comes off, one may feel hungry and be tempted to fill the vacated volume! Overall the digestive capacity is reduced because of the reduced abdominal volume. It is reasonable to assume that natural adaptation and anatomical flexibility deals with very tight corseting in the same way it accommodates pregnancy. To confirm this, it is now clear, that women or men who tightlaced were no less healthy than their uncorseted counterparts and this continues to be true today, regardless if they started early or late in life. Any person can become ill, regardless if they wear tight corsets or not. Well known tightlacers of the past, tended to have very good health and produced healthy children, confirming preservation of health and fertility.

For additional discussion on this subject visit the detailed medical section as well as Q&A at the LISA website. Corseting the Human Body by Dr. Ann Beaumont  provides detailed insight in the matter, but above all, the pressure produced by a corset with tight lacing is a comfort rather than a health question.

But, more than anything, observe the rule: "when the corset hurts, loosen the laces, because it is too tight." There is nothing else to it, but before it becomes painful,  it will very likely have been very uncomfortable long before it began to hurt. The low cost lingerie styles that are labeled as corsets are not proper corsets some don't even have busks or front and rear stays. If anything, their use would not promote corseting and they should not be worn, other than for decorative purposes.