Hi David,
Good choice; I find the extra mic positions very much worth having to give some extra options for the sound, and I always find myself using them.
Just to briefly describe the process of adding the full library in my experience:-
1. Once you've bought the piano, you'll get an activation code for the Full Library Upgrade.
2. You enter this activation code in eLicencer in exactly the same way you did for the first one, and it will create what it calls a Collection licence, which will include the original one ("SY Yamaha CFX") and the upgrade ("SY Yamaha CFX L2"). That's all you need for the licencing.
3. In MyVSL you'll find a new download file ("Yamaha CFX - Multi Library Download"). Downloading that file and opening it in the same Vienna Download Manager you already have installed will give you a list of the 5 new mics (and a couple of information files), and you can uncheck any you don't want to download from there. There original 5 mics will not be in this list.
4. You can download the mic files to wherever you want. Once downloaded, you run the installed in exactly the same way as you did previously for the standard libraries. You don't need to keep the download files afterwards, but if you want to keep them in case you need to reinstall at some point, then you can move them to wherever you want.
5. The installed additional microphone files will now be wherever you told the installed to put them. Again, these can be moved around to wherever you want, as long as you remember to add whatever folder(s) they're in to the Settings of Synchron Pianos when you next run it. Do that before clicking any preset.
6. Now you can run any preset to load the samples, and all 10 microphones will automatically be available for you.
That's it! It's very straightforward; much more so than most other piano VSTs.