Use Jbridge to bridge the older plugins if you are running 64bit 7.5.
Cubase has always given you backwards compatibility with older versions so you re-install SX, you will still have the license for it.
Grab the ‘Cubase plugins’ from SX, on PC it is at x86 Program Files/Steinberg/Cubase SX/VST Plugins and use Jbridge to bridge them to copy them to a new folder and call the new folder (compatibility) or something like that. Place the new folder in the 64bit VST plugin directory Program Files/VST Plugins. Now you have the missing SX plugins in Cubase 7.5 Open projects, see what plugins are missing, try to find installation files of those. I have done it to be able to open my older SX Projects, seems to work most of the time until I find a strange plugin. VSTi Instruments are more change the sound since SX, for example, you need the exact original version of Vanguard that you used in the project to get the same sound as they kept changing it in the updates. Audio plugins are less likely to change and I haven’t noticed anything. Auto Tune can be a problem, you need the older ones installed. You don’t need the original sound card to open the projects, just change the routing in VST Connections to that of your new sound card or use ASIO4ALL if you don’t have a new sound card.