Brownfield node upgrade strategy

An operator would like to develop brownfield SFU upgrade guidelines. They have access to different access technologies such as Fiber Deep and Fiber To The Home in their portfolio. Question is what are the guidelines for deploying technologies for different node densities?

When developing a long-term access network transformation plan, network operators tend to focus on demand growth as the key trigger for upgrades. While accommodating demand growth remains a key driver for upgrading a network, in reality other triggers may enforce a different upgrade strategy and alter the preferred upgrade path and the required investment profile.

An operator wants to move all subscribers to Fiber to the Home. He decides on an aggressive strategy to no longer invest in DSL technology upgrades but migrate network elements to FTTH when they need to be upgraded and wants to understand what is the best starting point: go with GPON or directly introduce XGS-PON?