Solution
Before you install or upgrade an NI-RFmx driver, ensure that you are willing and able to upgrade all your other NI-RFmx drivers to the same version. For example, if you already have installed RFmx SpecAn 1.2 and Demod 1.2 on your system, and you want to install SpecAn 2.0, you must also install Demod 2.0 at the same time or in a separate step. If you only upgrade SpecAn to 2.0 version, the Demod 1.2 API will be in non-working state. Refer to Figure 1.

Figure 1. Bad State resulting from partial NI-RFmx upgrade
If you upgrade one or more RFmx drivers, and want to downgrade to an older version, you must uninstall all of the RFmx drivers before reinstalling the older versions. If you do not uninstall all of the RFmx drivers before installing an older version, the newer version of the shared components installed by the newer RFmx driver will remain on the system and will be incompatible with the older versions of the drivers.

Figure 2. NI-RFmx downgrade process flow