Retire a SQL instance without impacting the primary server?
The goal is to release as many resource to the VM pool, as practical, pending the full server retirement.


You can just turn off sql server service of the instance that you are retiring and change the startup mode to disable.

Also, make sure to allocate the memory to other instance that is running form the decommissioned instance.

In a maintenance window, you can go ahead and uninstall SQL Server instance that you offlined.