The emergence of coherent pluggable interfaces is enabling to streamline the network architecture and is raising the prospect of extra reductions in both capital and operational expenditures. Nevertheless, the relatively long lifecycle of line systems (e.g., 10 years), when compared to the shorter lifecycle of line interfaces (e.g., 3 years), means that a large fraction of network operators will be looking to leverage these pluggable devices in brownfield deployment scenarios. In this case, the legacy line system can hamper the effectiveness of exploiting the pluggable line interfaces. This paper describes a comprehensive framework to estimate the impact of the line system on key network capacity and cost-related metrics when pluggable line interfaces are deployed. The framework is used on a reference optical transport network with growing demand for capacity and assuming a set of possible line systems, including legacy and modern ones. The simulation results highlight that although there are clear benefits in terms of usable capacity when these interfaces are deployed over modern line systems (greenfield scenario), it is still possible to reach competitive capacity figures with legacy line systems (brownfield scenario), particularly via exploiting a trade-off between capacity and the number of line interfaces used as regenerators.