Okay, I'm trying a build now. I also created a project from scratch so I could see what changed between versions.
I tried deleting ITrackableEventHandler but it was a little like pulling on a loose thread. Ran a diff on both directories to get a sense for how much difference there was between the two (is this listed somewhere else?) If not I'll include it here so anyone interested can extract the files that might be causing the conflicts.
I did try a version where I just deleted the whole scripts folder and reimported, that seems to have worked but I haven't tried a full build yet to see if it's in tact. If anyone else stumbles across these directions be sure to disable the auto-refresh in preferences or else you could end up in a very bad place.
Only in 2.6.7: CameraDevice.cs
Only in 2.6.7: DataSetTrackableBehaviour.cs
Only in 2.6.7: CylinderTarget.cs
Only in 2.6.7: DataSet.cs
Only in 2.6.7: ImageTargetBuilder.cs
Only in 2.6.7: ImageTracker.cs
Only in 2.6.7: GeometryStructs.cs
Only in 2.6.7: ICloudRecoEventHandler.cs
Only in 2.6.7: ILoadLevelEventHandler.cs
Only in 2.6.7: ITextRecoEventHandler.cs
Only in 2.6.7: ITrackableEventHandler.cs
Only in 2.6.7: ITrackerEventHandler.cs
Only in 2.6.7: IUserDefinedTargetEventHandler.cs
Only in 2.6.7: IVideoBackgroundEventHandler.cs
Only in 2.6.7: IVirtualButtonEventHandler.cs
Only in 2.6.7: Image.cs
Only in 2.6.7: ImageTarget.cs
Only in 2.6.7: Marker.cs
Only in 2.6.7: StateManager.cs
Only in 2.6.7: TargetFinder.cs
Only in 2.6.7: TextTracker.cs
Only in 2.6.7: Trackable.cs
Only in 2.6.7: TrackableBehaviour.cs
Only in 2.6.7: TrackableSource.cs
Only in 2.6.7: Tracker.cs
Only in 2.6.7: TrackerManager.cs
Only in 2.6.7: MarkerTracker.cs
Only in 2.6.7: MultiTarget.cs
Only in 2.6.7: QCARManager.cs
Only in 2.6.7: QCARRenderer.cs
Only in 2.6.7: QCARRuntimeUtilities.cs
Only in 2.6.7: QCARUnity.cs
Only in 2.6.7: VirtualButton.cs
Only in 2.6.7: Word.cs
Only in 2.6.7: WordList.cs
Only in 2.6.7: WordManager.cs
Only in 2.6.7: WordResult.cs
Files 2.6.7/DefaultInitializationErrorHandler.cs and 2.8.7/DefaultInitializationErrorHandler.cs differ
Files 2.6.7/DataSetLoadBehaviour.cs and 2.8.7/DataSetLoadBehaviour.cs differ
Files 2.6.7/CloudRecoBehaviour.cs and 2.8.7/CloudRecoBehaviour.cs differ
Files 2.6.7/CylinderTargetBehaviour.cs and 2.8.7/CylinderTargetBehaviour.cs differ
Files 2.6.7/DefaultTrackableEventHandler.cs and 2.8.7/DefaultTrackableEventHandler.cs differ
Files 2.6.7/GLErrorHandler.cs and 2.8.7/GLErrorHandler.cs differ
Files 2.6.7/ImageTargetBehaviour.cs and 2.8.7/ImageTargetBehaviour.cs differ
Files 2.6.7/Internal/BGRenderingBehaviour.cs and 2.8.7/Internal/BGRenderingBehaviour.cs differ
Files 2.6.7/KeepAliveBehaviour.cs and 2.8.7/KeepAliveBehaviour.cs differ
Files 2.6.7/MaskOutBehaviour.cs and 2.8.7/MaskOutBehaviour.cs differ
Files 2.6.7/MarkerBehaviour.cs and 2.8.7/MarkerBehaviour.cs differ
Files 2.6.7/MultiTargetBehaviour.cs and 2.8.7/MultiTargetBehaviour.cs differ
Files 2.6.7/QCARBehaviour.cs and 2.8.7/QCARBehaviour.cs differ
Files 2.6.7/SetBGCameraLayerBehaviour.cs and 2.8.7/SetBGCameraLayerBehaviour.cs differ
Files 2.6.7/TextRecoBehaviour.cs and 2.8.7/TextRecoBehaviour.cs differ
Files 2.6.7/TurnOffBehaviour.cs and 2.8.7/TurnOffBehaviour.cs differ
Files 2.6.7/TurnOffWordBehaviour.cs and 2.8.7/TurnOffWordBehaviour.cs differ
Files 2.6.7/UserDefinedTargetBuildingBehaviour.cs and 2.8.7/UserDefinedTargetBuildingBehaviour.cs differ
Files 2.6.7/VirtualButtonBehaviour.cs and 2.8.7/VirtualButtonBehaviour.cs differ
Files 2.6.7/WebCamBehaviour.cs and 2.8.7/WebCamBehaviour.cs differ
Files 2.6.7/WordBehaviour.cs and 2.8.7/WordBehaviour.cs differ
Only in 2.8.7: CylinderTargetBehaviour.cs.meta
Only in 2.8.7: CloudRecoBehaviour.cs.meta
Only in 2.8.7: GLErrorHandler.cs.meta
Only in 2.8.7: DataSetLoadBehaviour.cs.meta
Only in 2.8.7: DefaultInitializationErrorHandler.cs.meta
Only in 2.8.7: DefaultTrackableEventHandler.cs.meta
Only in 2.8.7: ImageTargetBehaviour.cs.meta
Only in 2.8.7: KeepAliveBehaviour.cs.meta
Only in 2.8.7: MarkerBehaviour.cs.meta
Only in 2.8.7: MaskOutBehaviour.cs.meta
Only in 2.8.7: MultiTargetBehaviour.cs.meta
Only in 2.8.7: QCARBehaviour.cs.meta
Only in 2.8.7: SetBGCameraLayerBehaviour.cs.meta
Only in 2.8.7: TextRecoBehaviour.cs.meta
Only in 2.8.7: TurnOffBehaviour.cs.meta
Only in 2.8.7: TurnOffWordBehaviour.cs.meta
Only in 2.8.7: UserDefinedTargetBuildingBehaviour.cs.meta
Only in 2.8.7: VirtualButtonBehaviour.cs.meta
Only in 2.8.7: WebCamBehaviour.cs.meta
Only in 2.8.7: WordBehaviour.cs.meta
Only in 2.8.7: Internal.meta
Only in 2.8.7/Internal: AndroidUnityPlayer.cs
Only in 2.8.7/Internal: AndroidUnityPlayer.cs.meta
Only in 2.8.7/Internal: BGRenderingBehaviour.cs.meta
Only in 2.6.7/Internal: CameraDeviceImpl.cs
Only in 2.6.7/Internal: CloudRecoImageTargetImpl.cs
Only in 2.8.7/Internal: ComponentFactoryStarterBehaviour.cs
Only in 2.8.7/Internal: ComponentFactoryStarterBehaviour.cs.meta
Only in 2.8.7/Internal: Qualcomm.Vuforia.UnityExtensions.dll
Only in 2.8.7/Internal: Qualcomm.Vuforia.UnityExtensions.dll.meta
Only in 2.8.7/Internal: VuforiaBehaviourComponentFactory.cs
Only in 2.8.7/Internal: VuforiaBehaviourComponentFactory.cs.meta
Only in 2.6.7/Internal: CylinderTargetImpl.cs
Only in 2.6.7/Internal: DataSetImpl.cs
Only in 2.6.7/Internal: IEditorCylinderTargetBehaviour.cs
Only in 2.6.7/Internal: IEditorDataSetTrackableBehaviour.cs
Only in 2.6.7/Internal: IEditorImageTargetBehaviours.cs
Only in 2.6.7/Internal: IEditorMarkerBehaviour.cs
Only in 2.6.7/Internal: IEditorMultiTargetBehaviour.cs
Only in 2.6.7/Internal: IEditorTextRecoBehaviour.cs
Only in 2.6.7/Internal: IEditorTrackableBehaviour.cs
Only in 2.6.7/Internal: IEditorVirtualButtonBehaviour.cs
Only in 2.6.7/Internal: IEditorWordBehaviour.cs
Only in 2.6.7/Internal: ImageImpl.cs
Only in 2.6.7/Internal: ImageTargetBuilderImpl.cs
Only in 2.6.7/Internal: ImageTargetImpl.cs
Only in 2.6.7/Internal: ImageTrackerImpl.cs
Only in 2.6.7/Internal: MarkerImpl.cs
Only in 2.6.7/Internal: MarkerTrackerImpl.cs
Only in 2.6.7/Internal: MultiTargetImpl.cs
Only in 2.6.7/Internal: NullWebCamTexAdaptor.cs
Only in 2.6.7/Internal: QCARMacros.cs
Only in 2.6.7/Internal: QCARManagerImpl.cs
Only in 2.6.7/Internal: QCARRendererImpl.cs
Only in 2.6.7/Internal: QCARUnityImpl.cs
Only in 2.6.7/Internal: StateManagerImpl.cs
Only in 2.6.7/Internal: TargetFinderImpl.cs
Only in 2.6.7/Internal: TextTrackerImpl.cs
Only in 2.6.7/Internal: TextureRenderer.cs
Only in 2.6.7/Internal: TrackableImpl.cs
Only in 2.6.7/Internal: TrackableSourceImpl.cs
Only in 2.6.7/Internal: TrackerManagerImpl.cs
Only in 2.6.7/Internal: VirtualButtonImpl.cs
Only in 2.6.7/Internal: WebCamImpl.cs
Only in 2.6.7/Internal: WebCamProfile.cs
Only in 2.6.7/Internal: WebCamTexAdaptor.cs
Only in 2.6.7/Internal: WebCamTexAdaptorImpl.cs
Only in 2.6.7/Internal: WordImpl.cs
Only in 2.6.7/Internal: WordListImpl.cs
Only in 2.6.7/Internal: WordManagerImpl.cs
Only in 2.6.7/Internal: WordResultImpl.cs
Only in 2.6.7/Internal: Wrapper
I've run into the same issues when upgrading our Vuforia Master Project. We keep a master on the asset server as we have written a few extension scripts and this (should) allow us to update vuforia into this project, check changes using the diff, before repackaging into our own.
Why was it decided to move so many of the original source files into .dll?