The evolution toward beyond-5G/6G services and applications requiring very high bandwidths will put ever-growing pressure on deployed optical networks to provide the spectral resources needed to accommodate such traffic loads. This is particularly true for the metro/regional segments, where expected traffic CAGR exceeding 40%, primarily driven by CDN traffic confined to the network segments closer to the end users, will push the fiber capacity to its fundamental limits on a number of "hot" links in the short to medium term, irrespective of the use of spectrally efficient modulation formats, especially suited for short-haul transmission. It is therefore critical to evaluate migration scenarios to expand spectral resources beyond the extended C-band (4.8 THz) mainly in use today. In this paper, we conduct multi-homed edge-to-core routing, modulation-level and spectrum-assignment simulations over Telefónica reference metropolitan networks, considering bandwidth-variable coherent transmission with per-channel capacity of 100 – 400 Gbps in 50 GHz and link-by-link C+L-band migration, while ensuring optimal GSNR performance. Results show that the rollout of L-band equipment over a reduced number of links can efficiently extend the lifespan of current networks, guaranteeing congestion-free operation with minimal intervention, and can be a suitable transitional capacity-stretching solution prior to undertaking more disruptive multi-fiber deployments.