In one environment, I had to test the possibility of an eRoom upgrade not succeeding. As part of the test, I backed up the three SQL databases, and performed the upgrade successfully. However, as a back out plan, I uninstalled the upgrade and removed the IIS folder and Program Files folder that were called eRoom. This means that the entire application is now uninstalled. Then I restored the databases. After rebooting the server and started to install the Index Server, the following error appeared:

The Indexing Server shares registry entries and files with the eRoom Server. You may not install an Indexing Server on an eRoom Server that is newer.

Resolution
To resolve this error, I removed the registry entries that are associated with the installation.
HKLM\SOFTWARE\eRoom
HKLM\SOFTWARE\Hummingbird
After removing the registry entries, the installation proceeded as normal.

On a side note, instead of creating a new eRoom, I elected to join an eRoom and selected the SQL server containing the restored databases. A message appeared indicating that a server of the same name existed, do you wish to continue. The response wass yes. eRoom installed successfully for this downgrade, backout plan.