A new project developed in NVivo 9.1 cannot be opened in an earlier version of QSR software, including NVivo 9.0, NVivo 8, NVivo 7, NVivo 2 or N6 (NUD*IST 6). QSR supports only forward compatibility of projects.
Projects developed using NVivo 9.0, NVivo 8, NVivo 7, NVivo 2, NVivo 1, N6 (NUD*IST), N5 or N4 software, can be opened and worked with in NVivo 9.1 however. Following this action, the original copy of the existing project remains unchanged. That means it can still be opened in the QSR software it was originally created in.
Refer to the Help in your NVivo 9.1 software for further information about how earlier projects are converted.