I saw the check box allowing the uninstallation of CC 2014 to be avoided, but decided to let it go ahead and uninstall older versions, forgetting that I would have to reinstall plugins. I did however take the precaution of creating a restore point in Windows 7, so I can roll the whole process back if I need to.
None of the plugins I use (Photokit Sharpener, Nik Collection, Topaz Remask, DxO Viewpoint) have released installers yet that work with CC 2015. However, as a temporary workaround, I did what Adobe says not to do, and just dragged the contents of the old CC 2014 Plugins folder to the Plugins folder for CC 2015. (CC 2015's old-version uninstaller left the old Plugins folders alone, probably so that we can see what we had there.) They all work fine in CC 2015. When the plugin makers release their new compatible installers, I'll go back and do it the *right* way, to avoid any potential problems in the future.
This approach may also work for others, possibly depending on which plugins you have.