Answer:
It is possible to import files from RFEM 5 into RFEM 6 or RSTAB 8 into RSTAB 9. Backward compatibility of RFEM 6 with RFEM 5, as well as RSTAB 9 with RSTAB 8, is not provided.
In general, you should save the RFEM 5 or RSTAB 8 model in the latest program version before the import.
Simply open the file from the previous version in RFEM 6 or RSTAB 9 with the menu command "File → Open".
Currently, it should be possible to import most of the model data and load cases and combinations, as well as the loads, into the new program series.
Unfortunately, this does not yet include objects such as releases, connections, entries in the add-on modules, components of intersections, generated wind and snow loads, and the printout report.
We strongly recommend checking all input data again during an import and supplementing them, if necessary.
