The primary difference between a Single container and the Multitenant Database container(MDC) is the SYSTEMDB. During the conversion from Single container to MDC a SYSTEMDB is created
This SYSTEMDB now has the following services nameserver, preprocessor, compileserver, daemon, webdispatcher for the entire HANA system(no INDEXSERVER service)
For each new Tenant Database created an INDEXSERVER service and the volumes are created
Due to the Architecture of SYSTEMDB and Tenant Database currently the conversion from MDC to Single container HANA system is not available but as per SAP Note 2104291 it is being considered by SAP for future versions
The picture from the below link helps to understand the difference in Architecture of different HAN systems in comparison to MDC
http://1h9mo84f79012h5z12640ehs-wpengine.netdna-ssl.com/wp-content/uploads/2015/01/mdc-1.jpg
The SAP note 2096000 is very useful to understand MDC configuration
Hope this helps