An organization on its process improvement journey may include process maintenance activities under the heading of process improvement. Most organizations do that. As far as I know, CMMI does not talk of process maintenance activities. To me, a few examples of process maintenance activities are those required for metrics data collection, project monitoring, process definition, making plans, etc. On the other hand, process improvement will include activities like metrics data analysis, identifying significant variations from planned arrangement, process re-definition based on improvement suggestions and metrics analysis, revising the plans in line to changing requirements, and taking appropriate preventive and/or corrective actions.
Are there any benchmarks for healthy process maintenance vs. improvement activities for software organizations?
These two concepts in inextricably intertwined. I would find it hard to believe that an organization does not identify any process improvement suggestions by just doing process maintenance. Even if you are just maintaining your Maturity Level, you will still identify improvements to the status quo. And I contend that even for process maintenance you have to perform data analysis, identify variations, and identify appropriate corrective actions, otherwise how would you be able to determine that you are maintaining what you have already achieved? I do not understand the need to define these two items as separate activities. What would be the point of merely maintaining your processes?