The developer's job is much easier if there is one simple roadmap for modernization. Unfortunately, such is not the case.
Many factors determine the wanted approach to modernization:
Any one of these items can be broken into multiple bullet points
Because there are different requirements, different starting points, different levels of support, and different levels of resource, there are many different approaches to modernization.
Although there are many approaches, one of the key challenges is to ensure that the chosen approach is fluid. One must walk the line between control and flexibility and develop the ability to embrace what is new while maintaining a working core.
At the outset, choosing a modernization path seems like a daunting task. If one accepts that the path changes when the process starts, it is much easier to get started.