The spatiotemporal aspects of the Portevin-Le Chatelier (PLC) instability are investigated, using infrared thermography, in an Al-4.5%Mg alloy loaded at room temperature in simple tension at an imposed strain rate such that a transition from type B to type C bands occurs during the test. By following the evolution in time of the band characteristics, the analysis reveals a close correlation between the dynamics of band nucleation and the organization of stress serrations during the type B-type C transition as strain increases. The initially small and regular stress drops become less regular, more dispersed and exhibit a larger magnitude across this transition. In the type B regime, new bands nucleate at constant frequency ahead of the previous one, with a band spacing slightly smaller than the bandwidth, giving an impression of continuous band propagation, although the bands are actually static. After the transition, the band spacing becomes much larger than the bandwidth, suggesting a trend to random nucleation in space. Similarly, the statistical distribution of the stress drops size evolves towards a more symmetrical peak-shaped Gauss-type distribution across the transition. These results are discussed in accordance with dynamic strain aging interpretations.