It is possible for the engineyard-serverside gem to force an upgrade during a deploy, if you are using the Engine Yard Cloud dashboard.
Although this rarely occurs since the engineyard-serverside gem is backward compatible, if a bug is introduced, the updated gem can prevent you from deploying your application.
If you are unable to deploy from the dashboard, it might be due to a bug in the version of engineyard-serverside that the dashboard is using. Try to deploy from the command line instead.
A command line deploy automatically changes the the version of engineyard-serverside on your instance to match the version you are using locally.
For more information on how to deploy from the command line, see Deploy From the CLI.
Special thanks go to Petteri Räty for reporting this issue.