The nature of the work does not preclude the organization from having to follow processes. The organization simply has to define the processes correctly for the different types of work and domains. For a Maturity Level 2 (ML 2) organization, this approach comes down to how the organization defines a project. In my experience, clients who are managing legacy and/or maintenance projects initially define individual changes as a project and it kills them because of everything required by Project Planning (PP) and Project Monitoring and Control (PMC). So the organization then has to think about how they manage work. At what level do they create a project schedule? Is it for each change? Or is it for a group of changes? Or is it on an annual basis? Basically the changes for maintenance work tend to be treated as a “job jar” and the organization pulls different jobs out of the jar to work on. So my clients in this situation, then consider work being managed annually, meaning that there is an annual project plan covering staffing levels, training, etc. That is, most of the PP Specific Practices (SPs). Then when they work on a specific job or task, they have a mini-schedule that is reviewed and compared to the annual plan to ensure that everything is compatible and they work to the mini-schedule using the annual project plan.
For legacy projects, the project plan, if it exists, was created well before the organization heard of or considered implementing a process improvement model like the CMMI. Consequently whatever exists was not developed per the current PP processes and there is no business value to go back and attempt to retrofit the old plan to the current process. What does make sense is for all project re-planning and re-baselining activities from this point forward to follow the new PP processes.