With the release of Dynamics NAV 2013 R2, the Classic report designer was discontinued, as was the whole NAV Classic Client. With each NAV version upgrade, custom objects (which includes reports) requires a special upgrade path.
Dynamics NAV 2009 versions (SP1 and R2) allow for storing both Classic and RDLC report layouts depending on which Client the user prefers to work with. Dynamics NAV 2013 and later allow only for the RDLC report layout. This means that in the course of upgrading reports from versions earlier than NAV 2009 to the version NAV 2013 or later, we need to use the Dynamics NAV 2009 version as an intermediary in order to bring our report to the new upgraded version.