Heroku supports multiple versions of the Play! Framework. You can specify which version you want in your dependencies.yml file. When you require the play framework put the version number after it:
# Application dependencies
require:
- play 1.2.4
If you don't specify a version the default version will be chosen for you.
When you're ready to upgrade from one version to another just update this version declaration. On your next deployment the new version of the framework will be used to build and run your app. This works for both upgrades and downgrades.
If your application is using the default version then it won't be updated when the default version changes. It will keep the version that was the default when it was first deployed. To take advantage of newer versions just change your dependencies.yml to specify the version that you want.