diff --git a/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift b/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift
index 5f6bd7682..cd2c4f620 100644
--- a/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift	
+++ b/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift	
@@ -72,23 +72,17 @@ class MachineDocument:
 			self.configureAs(analyser)
 			self.fileObserver = CSFileContentChangeObserver.init(url: url, handler: {
 				DispatchQueue.main.async {
-//					self.actionLock.lock()
-//					self.drawLock.lock()
-
 					switch(self.machine.effectForFile(atURLDidChange: url)) {
 						case .reinsertMedia:	self.insertFile(url)
 						case .restartMachine:
 							let target = CSStaticAnalyser(fileAt: url)
 							if let target = target {
-//								self.configureAs(target)
+								self.machine.substitute(target)
 							}
 
 						case .none:				fallthrough
 						@unknown default:		break
 					}
-
-//					self.actionLock.unlock()
-//					self.drawLock.unlock()
 				}
 			})
 		} else {
diff --git a/OSBindings/Mac/Clock Signal/Machine/CSMachine.h b/OSBindings/Mac/Clock Signal/Machine/CSMachine.h
index c2923804e..f78a33a83 100644
--- a/OSBindings/Mac/Clock Signal/Machine/CSMachine.h	
+++ b/OSBindings/Mac/Clock Signal/Machine/CSMachine.h	
@@ -79,6 +79,8 @@ typedef NS_ENUM(NSInteger, CSMachineChangeEffect) {
 - (void)setMouseButton:(int)button isPressed:(BOOL)isPressed;
 - (void)addMouseMotionX:(CGFloat)deltaX y:(CGFloat)deltaY;
 
+- (void)substitute:(nonnull CSStaticAnalyser *)machine;
+
 @property (nonatomic, strong, nullable) CSAudioQueue *audioQueue;
 @property (nonatomic, readonly, nonnull) CSScanTargetView *view;
 @property (nonatomic, weak, nullable) id<CSMachineDelegate> delegate;
diff --git a/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm b/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm
index 355278a99..ffb1a8162 100644
--- a/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm	
+++ b/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm	
@@ -34,6 +34,7 @@
 #include <atomic>
 #include <bitset>
 #include <locale>
+#include <memory>
 
 namespace {
 
@@ -50,6 +51,8 @@ struct MachineUpdater {
 	MachineTypes::TimedMachine *timed_machine = nullptr;
 };
 
+using Updater = Concurrency::AsyncTaskQueue<true, false, MachineUpdater>;
+
 }
 
 @interface CSMachine() <CSScanTargetViewDisplayLinkDelegate>
@@ -123,7 +126,7 @@ struct ActivityObserver: public Activity::Observer {
 	CSJoystickManager *_joystickManager;
 	NSMutableArray<CSMachineLED *> *_leds;
 
-	Concurrency::AsyncTaskQueue<true, false, MachineUpdater> updater;
+	std::unique_ptr<Updater> updater;
 	Time::ScanSynchroniser _scanSynchroniser;
 
 	NSTimer *_joystickTimer;
@@ -161,9 +164,11 @@ struct ActivityObserver: public Activity::Observer {
 			}
 			return nil;
 		}
-		updater.performer.machine = _machine.get();
-		if(updater.performer.machine) {
-			updater.start();
+
+		updater = std::make_unique<Updater>();
+		updater->performer.machine = _machine.get();
+		if(updater->performer.machine) {
+			updater->start();
 		}
 
 		// Use the keyboard as a joystick if the machine has no keyboard, or if it has a 'non-exclusive' keyboard.
@@ -191,6 +196,32 @@ struct ActivityObserver: public Activity::Observer {
 	return self;
 }
 
+- (void)substitute:(nonnull CSStaticAnalyser *)machine {
+	[self stop];
+
+	Machine::Error error;
+	ROM::Request missing_roms;
+	_machine = Machine::MachineForTargets(_analyser.targets, CSROMFetcher(&missing_roms), error);
+
+	_view.scanTarget.scanTarget->will_change_owner();
+	_machine->scan_producer()->set_scan_target(_view.scanTarget.scanTarget);
+
+	updater = std::make_unique<Updater>();
+	updater->performer.machine = _machine.get();
+	if(updater->performer.machine) {
+		updater->start();
+	}
+
+	Activity::Source *const activity_source = _machine->activity_source();
+	if(activity_source) {
+		_activityObserver.machine = self;
+		activity_source->set_activity_observer(&_activityObserver);
+	}
+
+	_speakerDelegate.machine = self;
+	[self.delegate machineSpeakerDidChangeInputClock:self];
+}
+
 - (void)speaker:(Outputs::Speaker::Speaker *)speaker didCompleteSamples:(const int16_t *)samples length:(int)length {
 	assert(NSUInteger(length) == self.audioQueue.bufferSize*(speaker->get_is_stereo() ? 2 : 1));
 	[self.audioQueue enqueueAudioBuffer:samples];
@@ -511,7 +542,7 @@ struct ActivityObserver: public Activity::Observer {
 }
 
 - (void)applyInputEvent:(dispatch_block_t)event {
-	updater.enqueue([event] {
+	updater->enqueue([event] {
 		event();
 	});
 }
@@ -727,10 +758,10 @@ struct ActivityObserver: public Activity::Observer {
 - (void)audioQueueIsRunningDry:(nonnull CSAudioQueue *)audioQueue {
 	__weak CSMachine *weakSelf = self;
 
-	updater.enqueue([weakSelf] {
+	updater->enqueue([weakSelf] {
 		CSMachine *const strongSelf = weakSelf;
 		if(strongSelf) {
-			strongSelf->updater.performer.timed_machine->flush_output(MachineTypes::TimedMachine::Output::Audio);
+			strongSelf->updater->performer.timed_machine->flush_output(MachineTypes::TimedMachine::Output::Audio);
 		}
 	});
 }
@@ -738,7 +769,7 @@ struct ActivityObserver: public Activity::Observer {
 - (void)scanTargetViewDisplayLinkDidFire:(CSScanTargetView *)view now:(const CVTimeStamp *)now outputTime:(const CVTimeStamp *)outputTime {
 	__weak CSMachine *weakSelf = self;
 
-	updater.enqueue([weakSelf] {
+	updater->enqueue([weakSelf] {
 		CSMachine *const strongSelf = weakSelf;
 		if(!strongSelf) {
 			return;
@@ -746,7 +777,7 @@ struct ActivityObserver: public Activity::Observer {
 
 		// Grab a pointer to the timed machine from somewhere where it has already
 		// been dynamically cast, to avoid that cost here.
-		MachineTypes::TimedMachine *const timed_machine = strongSelf->updater.performer.timed_machine;
+		MachineTypes::TimedMachine *const timed_machine = strongSelf->updater->performer.timed_machine;
 
 		// Definitely update video; update audio too if that pipeline is looking a little dry.
 		auto outputs = MachineTypes::TimedMachine::Output::Video;
@@ -785,7 +816,7 @@ struct ActivityObserver: public Activity::Observer {
 }
 
 - (void)stop {
-	updater.stop();
+	updater->stop();
 }
 
 + (BOOL)attemptInstallROM:(NSURL *)url {