Approaching a client with a suggestion to move all processes at once to another system, even in all respects superior to existing, is practically impossible without severe disruptions in existing processes.
However, having a number of structured and unstructured client processes, forms and workflows, it is always possible to run automatic or semi-automatic process discovery, which will collect data and metadata associated with these business artifacts. As a next step, you can run various process intelligence and process mining algorithms to reveal common patterns and most frequent relations in connected artifacts. Next, you can build a library of process fragments and entities, which comprise client’s business domain. This library will well illustrate, which of existing process modeling methodology is most close to what client really uses. Then, existing processes can be gradually reworked for compliance based on selected methodology and client vision.
This is a standard scenario working in any real BPM project for process integration and improvement. Standards serve here as a driver of structuring to crystallize existing experience in concise and compliant form.