Your question spans several distinct areas (sequential upgrade requirements, drain-out/parallel-run strategies, and process instance migration) so here are the most relevant docs to ground your planning. I found the following relevant resources:
- Upgrade to Camunda 8.9 - upgrading from an earlier version
- Version compatibility checks
- Planning a Successful Camunda 8 Upgrade
- Prepare for upgrade
- Migration journey - Drain out vs big bang
- Process instance migration concept
- Process instance migration user guide (Operate)
- Migrate process instance (REST API, single, 8.6+)
- Migrate process instances (REST API, batch, 8.8+)
- Versioning process definitions best practices
- Forum: Migration from 8.1.5 to 8.5.17
- Forum: Camunda Upgrade from 8.3 to 8.6
- Forum: How to Migrate Data from Camunda 8.2 to Camunda 8.8?
Does this help? If not, can anyone from the community jump in? 
Hints: Use the Ask AI feature in Camunda’s documentation to chat with AI and get fast help. Report bugs and features in Camuda’s GitHub issue tracker. Trust the process. 