The Installer should NOT EVER DELETE the DL'ed files. It sounds like this is default behavior, and needs to get fixed or at least have an option to not do so!! This is my first dance with C3.
New updates should be named appropriately with a date and/or version info, if needed. My ISP is really lousy, so I don't want to have to deal with DL'ing the files again if something happens.
Not to hijack, but I just started the C3 for my SonarPro DL's and then wondered where they were getting put. My C: drv is a small SSD (120GB). I certainly don't use my SSD for temp DL storage in any event, even if W8.1 seems to thinK 'Progam Data' needs to be on 'C:'. I ended up aborting the install, re-configuring the paths and re-starting. I know that I should have done the config first. In hindsight, I am a bit surprised that the defaults weren't setup from a user dialog upon initial installation rather than using some 'defaults'. It took a while to 'see' the 'cog' and of course, the installation needs to get aborted to update them. It may be a common icon, but what it has to do with 'Settings' is beyond me.
The existing DL'ed files from a partial install don't get transferred to the new directory either. So, interruptions require starting over as well. If I spend time DL'ing then I want the files to persist after the installation. Particularly, since the C3 doesn't appear to be too smart about the whole DL process. I always keep my DL installation files on a different drive than the DAW drive anyway, so I completely agree with the OP.