Can you have multiple versions (builds) of the Flex Viewer software installed on the same server?
If you have an older version of Flex Viewer on a server and then install a newer version, will only the new version be available to the Flex Viewer applications stored on that server?
Depending on how you are deploying your Flex Viewer to your web server... You can have as MANY versions of FV on the web server as you want, as each should have it's own URL. For example I have 2.4, 2.5 and 3.0 all deployed on my server.
This is done through personal good practice and convention. I name my projects with the FV version number in the project name and when I deploy them I do the same with the virtual directory name.
Download a particular version of the FV source code and unzip that in a folder called "Flex Stuff" to a sub folder called something like "flexviewer-3.0-src".
I have my Flash Builder workspace set to c:\Flash Builder 4.5.1
I then copy the contents of the "flexviewer-3.0-src" folder from step 1 to a new folder in "c:\Flash Builder 4.5.1" called "FlexViewer3.0"
Now the project is ready to import to FlashBuilder.
In Flash Builder go to the file menu and click "Import Flash Builder Project..." and then choose "Project folder" and browse to the folder in step 3.
Then all you have to do is add the agslib-3.0-2012-06-06.swc to the projects libs folder and build the project.