Wavelength conversion has been shown as one of the key techniques to improve blocking performance in a wavelength-routed all-optical network. Given that wavelength converters nowadays remain very expensive, how to make effective use of a limited number of wavelength converters becomes an important issue. In this paper, we propose Sparse-Partial Wavelength Conversion (SPWC) network architecture with the inherent flexibility that can facilitate network carriers to migrate the optical backbone to support wavelength conversion. We demonstrate that this network architecture can significantly save the number of wavelength converters, yet achieving excellent blocking performance. Theoretical and simulation results indicate that, the performance of a wavelength-routed WDM network with only 1-5% of wavelength conversion capability is very close to that with Full-Complete Wavelength Conversion capability.