How the successively upward, isolated source-dominated propagation of the intracloud (IC) lightning transitions into highly branched, sideways propagation remains an intriguing question. Because the initial IC leader development is usually obscured by thunderclouds, there are few reported optical observations of the initiation and early propagation of IC lightning (Stolzenburg et al., 2021). Here, we analyze and detail the observations of this transition during initial IC leader development with data from optical instruments (Atmosphere-Space Interactions Monitor on the International Space Station), LF magnetic sensors, and VHF interferometry. This transition stage is initially defined by characteristics of the VHF interferometry source maps. By comparing multiple measurements for the same flashes, we find that this transition stage is also defined by repeatable (but different) features in the LF power density and optical waveforms. We find that the ratio of 337 nm (blue)/777.4 nm (red) optical radiance is above unity prior to the transition but is almost always below unity after the transition. The variance in this optical ratio suggests that the dominant illuminating process changes from isolated streamer activities (blue) to thermal channel excitations (red) through the transition. Although the decrease of the optical ratio after the transition could result from the extension of the hot leader channel, we find that the blue radiance drops through the transition, while the red radiance remains almost invariant. Furthermore, the optical radiance reaches the maximum when the transition starts and the LF power density sharply decreases after the transition, suggesting the transition may occur when the leader gradually propagates outside of the high E-field region.