I came across following situation:
Customer installed two SQL server nodes in Failover cluster with six SQL instances. During the installation they used by mistake Evaluation Edition.
After six months both SQL server licenses expired and after OS patching&reboot all SQL instances ended up in Failed state, not able to start(Databases service resource).
We tried Microsoft recommended approach. Running SQL installer, Maintenance and Edition Upgrade. Unfortunately Edition upgrade was showing below error:
We decided to build two new SQL servers(now with proper Product Key, not Eval), joined them into existing SQL cluster and ran Add Node to Failover Cluster for each instance. After we moved all failed SQL instances on new SQL nodes and all SQL cluster resources(including Database services) started successfully.