If symptom displacement happens, can that lead to a more serious and intractable malady, or will the Law of Karma ensure it is only something of comparable severity?

The Law of Karma is usually very precise in meting out consequences for unfinished karmic business, but that is a dispassionate mechanism and may not take into account individual perceptions and sensitivities. So a person might have a particular dislike for one kind of impairment or discomfort than another that might be judged as a similar degree of discord giving rise to symptoms, but not so tailor-made to an individual idiosyncrasy that it will take into account a particular person's heightened fears or oversensitivity to certain kinds of adverse symptoms. Many things can be observed to change characteristics and may well be perceived as a worsening. In some cases, there may be other factors involved that are also unappreciated, for example, that the attempts made to fix a problem, say through a medical treatment, are ramping up inner fears about being incapacitated or in a weakened and vulnerable state, and this will further inflame old karmic business that might pile on. So there can be quite a wide range of possibilities in a very dynamic and shifting landscape that will be difficult to understand, in terms of where it all is coming from and what factors are determining the precise nature of the changing manifestation of karmic negativity. Given the complexity, many things can happen that might not be seen by third parties as comparable, but a worsening of things.