All of our installers install the full product. They do not make any changes to an existing version. If you want to run the 64-bit of SlickEdit 2011, download the installer and it will install into a new directory.
The 64-bit version is provided at no additional charge to those running on Windows or Linux. So, if you purchase an upgrade license for SlickEdit 2011, you will be able to download both the 32-bit installer and the 64-bit installer.
The two versions are functionally identical. There are some issues with using SCC providers in the 64-bit version, though. Theoretically, a 64-bit application should run faster on 64-bit hardware than the same application running as a 32-bit application. However, we don't have any specific timings to back that up.