You can chose the platform that your modules plug into, including what that platform contains.
From the Properties/Libraries tab, you can change the Java Platform, the Netbeans platform (usually you have only one of each installed but you can install more, especially if you're testing whether code works on multiple platforms). The clusters and modules you select are included in the platform you plug into. If you exclude modules required by your suite, the GUI complains (in NetBeans 6 this is red text at the bottom). Follow the directions of the red text by adding modules until there is no more red text.
If you have both Netbeans 5.5.1 and Netbeans 6 Beta 2 installed (this may happen with other combinations as well), you may have the following strange problem: If you try to change the platform of a project suite from 5.5.1 to 6.0, you find that the platform refuses to change. Uninstalling Netbeans 5.5.1 solves this problem. There must be better solutions, however.