So long as you are following best practices, migrating an existing Drupal site onto PANTHEON is not very difficult. This page will walk you through the steps you need to take to deploy an existing site onto the base installation. If you are interested in deploying from version control, please see deploying from VCS with PANTHEON.
First Things First
This documentation assumes that you haven't hacked core. :)
If you haven't already, you might also want to change the root mysql password at this time. Instructions for that are in the README.txt on your server.
Next you'll need to scp (or use an sftp client like Transmit, Filezilla, or the like) a mysql export (dump) of your existing site, and the contents of your files directory as well as any contributed/custom modules and themes you run, onto the server. You should load the dump over the top of the pressflow database (including updated credentials as needed):
mysql -u root -p pantheon < my_site_dump.sql
Then place your contrib modules in /var/www/pressflow/sites/all/modules and themes in sites/all/themes, like so:
cp -R my_site_modules/* /var/www/pressflow/sites/all/modules/
cp -R my_site_themes/* /var/www/pressflow/sites/all/themes/
Lastly, place the files directory wherever your Drupal application expects it, which should probably be in
sites/something/files. Then reset the ownership of this directory so Apache can continue to maintain it. Assuming the location you expect is sites/all/files:
chown -R www-data /var/www/pressflow/sites/all/files
If you did not previously have the ApacheSolr and Varnish modules enabled, you will need to enable them, and configure the ApacheSolr module to use the "default" search database.
You should also confirm that the paths specified under admin/settings/file-system are valid and writeable by the www-data user. If not, you'll see a warning on that page.
The last bit of configuration is to log in, go to
admin/settings/performance, and set caching to external, define a max page age, and be sure that page compression is off. This will allow Varnish to start picking up whole pages for your logged-out users, as well as letting the lower-level mod_deflate do the work of compressing your pages for transport.