Experimental or calculated phase diagrams and field observations demonstrate that the same system under different but similar physicochemical conditions often exhibits many distinct phase assemblages or underwent distinct reaction paths. In order to analyze the reason for this phenomenon, the oxygen fugacity-silica activity [log f(O2)-loga(SiO2)] phase diagrams of the CaO-FeO-SiO2-O2 system (fayalite, kirschsteinite, hedenbergite, magnetite, andradite, hematite and wollastonite) are calculated under different temperatures (T) and pressures (P). The results discover such a phenomenon that during the variation of constraint conditions, the phase diagrams of two one-grade multisystem (i.e., the subsystems consisting of an invariant assemblage plus an additional phase) undergo change in topological configuration in sequence; in either change, the two invariant points on each univariant line of the same one-grade multisystem approach each other first, then coincide at one point, and then move away from each other in their original directions. Consequently, all relatively stable invariant points under the original conditions become metastable as a whole under new conditions, and all metastable invariant points under the original conditions simultaneously become relatively stable as a whole under new conditions, where the stable part and metastable part in each configuration are complementary in phase relations. As a result, the topological configurations of the whole one-grade multisystem phase diagram (including both stable and metastable parts) exhibit mirror symmetry. Such changes, particularly their conditions, should be common for the phase diagrams of multiphase, multicomponent systems under specific constraints. It is accompanied by abrupt change of a series of phase assemblages and reactions, and thus can influence the evolutional paths of relevant geological processes, while the conditions for one or multiple configurational changes can be used to determine the range of the constrained variables for the change of phase assemblages, and their calculation only involves the phases in a one-grade multisystem (rather than the whole system). Note that a "relatively" stable invariant point in a one-grade multisystem may become metastable in another one-grade multisystem. Therefore, it is possible that only partial "relatively" stable invariant points in a one-grade multisystem are still stable in a higher-grade multisystem. In addition, the change of constrained variable often leads to the occurring of new phase(s) or the disappearance of the original phase(s), and this change may make the original one-grade multisystem evolve into a new system before its phase diagram configuration changes, which significantly reduces the chances for the original one-grade multisystem phase diagram to change its configuration. In this article, the phenomena mentioned above are well analyzed and explained by the topological analysis theory of phase diagrams.