Current fixed grid DWDM networks are expected to be insufficient to accommodate expected demands in few years. Hence, operators need to compare different alternatives to cope with future demands. This paper presents a step-by-step techno-economic methodology to compare and identify the most convenient upgrade alternative(s). One of the possible migration scenarios is towards flexible grid optical networks. The comparative evaluation is made in terms of investment and operational expenditures considering the cost per spectral unit. The proposed methodology is applied to an operator having as initial scenario a statically operated network using a fixed grid with connections at 10 Gbps. Three different upgrade alternatives are evaluated: (i) static operation using the fixed grid is maintained but the bit rate of connections is increased to 100 Gbps; (ii) the network is operated dynamically (lightpaths established/released on demand) in a fixed grid with 100 Gbps connections and finally, (iii) a dynamic network operating with flexible grid. Results for this particular case, show that dynamic operation with flexible grid is not the most convenient migration alternative, given the high cost of the required equipment. This methodology is expected to be useful for network operators evaluating different upgrade alternatives for their infrastructure, and manufacturers aiming to evaluate the potential of their equipment.