For the long-term sustainability of a new system and graceful migration to it, make sure to prepare proper documentation and support. Document your coding standards, which everyone can follow and which helps to keep the new system up to date. Keep your architecture documents as working artifacts and keep them updated as technology trends change. Keeping your system up to date will ensure that you don't fall into the legacy system modularization situation again.
Prepare a comprehensive runbook to support new and old systems. You may want to keep the old system for some time until the new system can accommodate all business requirements and run satisfactorily. Update the support runbook, and make sure that you don't lose knowledge due to employee attrition, and that the overall knowledge base is not processed in a people-dependent manner.
Keeping track of system dependencies helps you to determine the impact of any changes in the future. Prepare training...