Multicast routing research efforts have mostly focused on supporting the host-group model in which multicast packets are addressed to a host (or multicast) group. Another multicast routing approach uses multi-destination addressing, where a multicast packet carries a list of the unique (unicast) addresses of all the group members. This form of routing can be accomplished using limited or no additional state beyond the existing unicast routing tables. It, therefore, scales well with the number of multicast sessions but does not scale well with the size of the multicast group and, in fact, requires the size of the multicast group to be below a certain threshold. In this paper, we envision a future scenario in which both host-group and multi-destination addressing routing approaches coexist within the Internet. We develop a dynamic routing context for this future scenario wherein a multicast session can adapt among different routing configurations depending on the number of multicast group members and how this number changes over time. We consider three routing options: (1) A single multi-destination addressed flow – suitable for small-group sessions, (2) multiple multi-destination addressed flows – suitable for medium-group sessions and (3) a single host-group addressed flow – suitable for large-group sessions. For multicast sessions that vary in group membership over time, different routing protocols may be best at different points in time. Our work is concerned with the development and evaluation of protocols that allow a multicast session to dynamically switch among these three routing options as the size of the session changes.