When you delete the source video files (of a previously imported AVCHD video) in "iMovie Events", existing iMovie projects referring to those source files become unusable for further re-encoding / processing. But don't worry, keep the project files (even delete the media browser shared files for it). You can reactivate the project later again if you still have the original AVCHD source files (with the full folder structure as on the memory card). You can later import again the original AVCHD files (that get converted again newly to AIC). Make sure that the event name is the same as before. then it should work out of the box. If you do not remember the name of the event folder used in the project, you can see all the clips used in the project by opening the package contents of the project and opening the file called "Project" with the "Property List Editor" (need to install the Developer Tools to have that), it is a binary plist, which contains all the full path names to the clips (including the name of the event sub-folder).
To make it easier in the future, use the same name for your archive (e.g. on DVD or as DMG) as for the event name in iMovie.
This whole thing is especially handy because the AVCH source files are taking around ten times less space than after they are imported and converted to AIC (Apple Intermediate Codec). No need to buy all the time new hard drives when you want to keep all the projects in iMovie editable, but the events keep up using up the hard drive space so quickly for imported AVCHD footage.
Also remember that in order to be able to import the video again to iMovie, you need to have the full folder structure as on the memory card or hard drive, therefore back the hard drive or memory card to a DMG or DVD / Blue Ray drive with the complete folder structure.
This tip might even work with DV files, imported from DV Camcorders, but I haven't tested it yet.