Coherent transmission is becoming pervasive in core transport networks. While current deployments in these networks consist mostly of 100 Gb/s channels, future ones will also include single carrier 200 Gb/s channels and superchannels for 400 Gb/s and beyond, all of which in coherent formats. Meanwhile, metro networks also have started to evolve from directly to coherently-detected modulation formats, benefiting from the reduced costs associated to higher volume production of coherent 100 Gb/s transponders. As both metro and core networks will soon converge to coherently-detected formats, this will facilitate exploiting transparent handover of wavelength services between both network segments to reduce overall capital expenditures, especially in scenarios where the same operator has a core network to provide interconnectivity between several metro networks. This paper overviews how traffic handover between the metro and core is currently realized, identifies challenges associated to using transparent handover and projects how technology evolution will enable to mitigate some of them.