Hi,
ArchiCAD 8.1 library is only slightly modified compared to 8. Therefore, you can continue ArchiCAD 8 files in 8.1 by just loading the ArchiCAD 8.1 library.
Feature developement and bugfixing in upcoming major releases may require more significant modifications in objects, therefore we cannot guarantee that the new objects will behave exactly the same as in previous versions.
Therefore, we always recommend to keep a backup PLA file saved in the original version, so you can go back to it anytime if you might need it.
Gergely Kmethy
VP, Customer Success, Graphisoft