The mechanisms of endosome biogenesis and maintenance are largely unknown. The small GTPases Rab5 and Rab7 are key determinants of early and late endosomes, organizing effector proteins into specific membrane subdomains. Whether such Rab machineries are indefinitely maintained on membranes or can disassemble in the course of cargo transport is an open question. Here, we combined novel image-analysis algorithms with fast live-cell imaging. We found that the level of Rab5 dynamically fluctuates on individual early endosomes, linked by fusion and fission events into a network in time. Within it, degradative cargo concentrates in progressively fewer and larger endosomes that migrate from the cell periphery to the center where Rab5 is rapidly replaced with Rab7. The class C VPS/HOPS complex, an established GEF for Rab7, interacts with Rab5 and is required for Rab5-to-Rab7 conversion. Our results reveal unexpected dynamics of Rab domains and suggest Rab conversion as the mechanism of cargo progression between early and late endosomes.