I should have thought a bit more before posting that. While it will work, there is a much simpler way: Create a shortcut to the .QDF file you want Quicken to open.
And when you asked if you could "erase Quicken's memory" of the last used file, I spoke too soon.
With Notepad - or similar; any file editor that will NOT make any changes (such as line-breaks or page breaks) you do not specifically intend: Open C:\Users\YourWindowsUserName\AppData\Roaming\Quicken\Config\quser.ini At the beginning of the file, you should see something like this (the file names are fake):
[RecentFiles] 1=C:\QuickenDataFile-1 2=C:\QuickenDataFile-2 3=C:\QuickenDataFile-3 4=C:\QuickenDataFile-4Those would be the last 4 files you opened in Quicken (which you see at the foot of the Quicken File menu). You can remove the file names - but leave the number and the equal sign - then Save the file, and Quicken will have no memory of the file(s) it had open. The end result should look like this:
[RecentFiles] 1= 2= 3= 4=