When changing an indexed column in a list from a single response to allowing multiple responses, an error occurs:
"The type of this column cannot be changed because it is currently being indexed"
This itself is expected behavior. However, after doing so, going back to the list will show that any metadata for that column is no longer visible.
If multiple selections are chosen for a column/field, then the ability to index that column will not be available (by design).
After reproducing the issue, the metadata in the "Assigned To" column appears to be missing.
Browse to ‘Settings | List Settings’ and click on ‘ Indexed Columns’
"Assigned To" is still set to being indexed, so uncheck it and click ‘OK’
Go back into the list (note that that the "Assigned To" column still does not contain metadata).
Then go back to ‘Settings | List Settings ‘ | choose the ‘Assigned To’ column and toggled 'Yes' for "Allow for multiple selections".
Go back to the list and notice the metadata is back for ‘Assigned To’ column.
Now users can make the choice: Leave this value as a multi-value field or set back to a single value and then add back as an Indexed column.