Social interaction is a continuously unfolding process in which partners adjust their coordination in response to changing goals, task phases, roles, strategies, and ongoing feedback. Such coordination is also shaped by prior interaction experience. Inter-brain synchrony (IBS), defined as the temporal coordination of neural activity across individuals during social interaction, provides an important window into the neural mechanisms underlying interpersonal coordination. Existing research shows that IBS varies across interaction contexts and changes over the course of an interaction. Prior interactions, repeated training, and external interventions may also influence subsequent patterns of IBS. This review synthesizes evidence for these changes and their relationships with behavioral coordination and interaction outcomes. It conceptualizes IBS flexibility along two interrelated dimensions —adaptability and experientiality—distinguished primarily by the factors on which IBS changes depend. Adaptability refers to dynamic adjustments in IBS in response to current interaction goals, task phases, role allocation, strategies, and feedback. These adjustments may manifest as increases or decreases in synchrony, transitions between distinct IBS states, or reconfiguration of inter-brain networks. Experientiality refers to the influence of prior interaction experience or external interventions on subsequent IBS. Such effects may manifest as post-task residual synchrony, changes across repeated interactions or training sessions, and alterations induced by noninvasive brain stimulation or multi-brain neurofeedback. Changes within an ongoing interaction may reflect both current interaction demands and accumulated experience. Identifying experience-dependent effects therefore requires distinguishing the phase in which experience is acquired from a subsequent assessment phase, while accounting for current interaction demands. Much of the available evidence comes from cross-sectional comparisons or assessments at only two time points; direct evidence of persistence, recurrence in later interactions, or generalization across partners and contexts remains limited. At the mechanistic level, several processes may contribute to these two dimensions of flexibility. Metastability may serve as an overarching dynamical principle, allowing interactive systems to shift between integration and segregation rather than remain in a fixed state of high synchrony. Within this framework, adaptability may be driven by mutual prediction and prediction-error updating, which support real-time changes in coupling strength and inter-brain state transitions. Experientiality may reflect history-dependent reconfiguration of this metastable system through accumulated co-occurrence, relational learning, reactivation, offline consolidation, and selective retention of functionally relevant coupling patterns, thereby reshaping subsequent coupling tendencies, state-transition patterns, and recurrent coordination configurations. It should be noted that these proposed mechanisms draw primarily on theoretical frameworks derived from research on learning and plasticity within individual brains and require validation in future hyperscanning studies. This review also summarizes quantitative measures relevant to the two dimensions of IBS flexibility. Adaptability can be captured by temporal-dynamic indices, such as sliding-window trajectories, variability, state transitions, directionality, time-lagged coupling, and dynamic network organization. Experientiality can be quantified using measures of post-task residual coupling, pre-post differences in IBS, delayed retention or reinstatement, and longitudinal trajectories of growth or maintenance. These measures provide statistical descriptions of IBS patterns but are insufficient, on their own, to support inferences about adaptability or experientiality. Such inferences require linking observed patterns of IBS change to specific interaction events and behavioral changes. Guided by this framework, future research should combine dynamic analyses, tracking at multiple time points, and experimental manipulations to examine how IBS flexibility relates to interaction demands, behavioral adjustments, and interaction outcomes, as well as whether experience-related effects are retained or transferred across different interaction contexts. Such work may deepen our understanding of the neural mechanisms underlying dynamic interpersonal coordination and provide a theoretical basis for research and applications in educational and clinical settings.