Cost-effective short-haul but high-capacity optical transport systems are becoming increasingly important for metropolitan-area and access applications; examples are metro-feeders, inter- or intra-office links between routers and cross-connects, and storage-area networks. Transmission links of this kind are characterized by distances of 10 to 100 km and typically use directly modulated lasers providing a high output power at a low device cost. Due to the inherent wavelength drift of uncooled system components, full spectrum CWDM is defined for up to 16 or 18 channels on a coarse 20-nm wavelength grid between 1270 and 1610 nm. To meet the demand for the increase of transmission capacity in the metro and access domain, there are several potential strategies to remove the capacity limit of 16×2.5 Gb/s found in conventional CWDM systems while still retaining the cost-effectiveness and the modular "pay as you grow" philosophy of CWDM. In this paper, we provide an overview of recent progress in the field of CWDM upgrade techniques. CWDM architectures with increased per channel bit-rates and sub-band DWDM channel overlays are reviewed and experimental results are presented. The particular combination of CWDM with DWDM sub-bands and bit-rate upgrades is expected to deliver the highest capacity gain. However, we also show that the suitability of the system for higher bit-rates hinges on the following considerations: the availability of low-cost 10-Gb/s DMLs at the respective wavelengths, the link budget and the chromatic dispersion of the deployed system.