BPD patients may occasionally show apparent remission or normalcy in traits such as neuroticism, while hysteric or depressive personality disordered-patients will manifest these traits more consistently. This also highlights the level of 'hope' one should have about what is seen as an improving sign during treatment. While a marked reduction in neuroticism might be a sign of responsiveness in a depressive personality type, in a BPD patient it may merely be another manifestation of the illness, part of the BPD cycle, of finding someone or someone to fixate upon to ease the patient's lack of a sense of core identity. In particular, neuroticism and conscientiousness "showed greater mean-level change, with neuroticism declining faster and conscientiousness increasing faster, in the BPD group" as compared with other traits in the FFM (Hopwood 2009, p.806). BPD's controversial nature as a diagnosis is not seriously disputed by the authors, and opponents of the diagnosis might contend that the method is tautological: the initial diagnosis of all study participants is accepted, and the methods used are merely personality inventories of a typical 'multiple choice' format for character stability that assume the consistency of all of the patient's diagnosis. However, the assessment was ipsative (self-reported) as well as based upon pre-existing diagnosis by a clinician. The longitudinal nature of the study and the multiplicity of the inventories argue in favor of the researcher's thoroughness.

Instability is already built into the diagnosis. The diversity of BPD characteristics may also be rooted in the uncertain history of BPD in clinical literature -- once considered on the borderline of psychosis and neuroticism, now BPD is viewed as an impulse disorder that may be exacerbated by trauma (such as sexual abuse) and problems with the central nervous system regulation of serotonin (Oldham 2005). Ultimately, this study seems to mainly confirm that individuals who receive a diagnosis of BPD are more likely to show trait instability, which the authors see as the result of the illness, or which could be read as a product of clinicians using BPD as a 'garbage can' diagnosis for a wide variety of difficult-to-treat anxiety, depressive, and impulse-control disorders.
References

Hopwood, C.J. (et al. 2009, November). The stability of personality traits in individuals with borderline personality disorder. Abnormal Psychology, 118(4):806-15.

Oldham, John M. (2004, July 1). Borderline personality disorder: An overview. The Psychiatric

Times. Retrieved July 19, 2010 at http://www.psychiatrictimes.com/borderline-personality/content/article/10168/53976