This is what I would do..
Install sdks in a separate directory say <DRIVE>/gwt-sdks/2.0 , 2.1,
1.7 etc
In Eclipse under Preferences, just choose which one you want to be
active/default. Same goes for app engine
Now under project properties, chose the desired sdk in "Use Specific
SDK"
If you reference versioned gwt jars in your project, then you better
mavenize it if possible and change dependency as need be
best
KD