You can do the downgrade manually by download do it by downloading the SDK.
Try this link https://code.google.com/p/googleappengine/downloads/detail?name=appengine-java-sdk-1.7.5.zip&can=2&q=label%3AOpSys-All
Then you have to add it to Eclipse by going into:
Window > Preferences > Google > App Engine.
Then you click on the "Add" button, and you add the folder where you downloaded the old SDK
Once added you have to configure the project by going into:
Properties > Google > App Engine, and there you select the 1-7-5 SDK
And you should be all set