OK so you produce the 'roadmap' for how to start to integrate the new systems into the old - could be cloud based, could be crowbar into the internal setup, could be complete new redesign of internal. All of which make for good roadmap's - people like this roadmapping - you can explain what you are up to with a roadmap. The interesting part is what you start up the engine and begin to drive using the 'roadmap'.
What you find is that it is difficult to run a programme/project just using a 'roadmap' - the signposts are there but the details of what awaits you on the roads is what is needed when you drive. Bends, hollows, potholes (just driven down a few of them), pedestrians, cyclists all sorts of hazards that can only be negotiated on-the-fly. No amount of project (re)planning will help you much either - though you will probably be able to frighten yourself with how much all this dodging is costing you.
Sounds like you need to be 'agile', particularly if you have a strict deadline looming, but what does agile mean? Can a large programme actually be agile? Haven't we been there before with Rapid Application Development (RAD), and other associated methods?
Or is it the other definition of RAD - Rapidly Approaching Disaster - that will be the norm?
The drive continues.....