diff --git a/server/src/imfs.rs b/server/src/imfs.rs
index a1190a15..21f6be1e 100644
--- a/server/src/imfs.rs
+++ b/server/src/imfs.rs
@@ -105,6 +105,16 @@ impl Imfs {
         Ok(())
     }
 
+    pub fn get_root_for_path<'a>(&'a self, path: &Path) -> Option<&'a Path> {
+        for root_path in &self.roots {
+            if path.starts_with(root_path) {
+                return Some(root_path);
+            }
+        }
+
+        None
+    }
+
     fn remove_item(&mut self, path: &Path) {
         if let Some(ImfsItem::Directory(directory)) = self.items.remove(path) {
             for child_path in &directory.children {
diff --git a/server/src/path_map.rs b/server/src/path_map.rs
index f1907b30..ebb23364 100644
--- a/server/src/path_map.rs
+++ b/server/src/path_map.rs
@@ -1,5 +1,5 @@
 use std::{
-    path::{Path, PathBuf},
+    path::{self, Path, PathBuf},
     collections::{HashMap, HashSet},
 };
 
@@ -70,4 +70,27 @@ impl<T> PathMap<T> {
 
         Some(root_value)
     }
+
+    pub fn descend(&self, start_path: &Path, target_path: &Path) -> PathBuf {
+        let relative_path = target_path.strip_prefix(start_path)
+            .expect("target_path did not begin with start_path");
+        let mut current_path = start_path.to_path_buf();
+
+        for component in relative_path.components() {
+            match component {
+                path::Component::Normal(name) => {
+                    let next_path = current_path.join(name);
+
+                    if self.nodes.contains_key(&next_path) {
+                        current_path = next_path;
+                    } else {
+                        return current_path;
+                    }
+                },
+                _ => unreachable!(),
+            }
+        }
+
+        current_path
+    }
 }
\ No newline at end of file
diff --git a/server/src/rbx_session.rs b/server/src/rbx_session.rs
index ce93d6d7..0181267a 100644
--- a/server/src/rbx_session.rs
+++ b/server/src/rbx_session.rs
@@ -13,7 +13,7 @@ use crate::{
     message_queue::{Message, MessageQueue},
     imfs::{Imfs, ImfsItem, ImfsFile},
     path_map::PathMap,
-    rbx_snapshot::{RbxSnapshotInstance, reify_root},
+    rbx_snapshot::{RbxSnapshotInstance, reify_root, reconcile_subtree},
 };
 
 pub struct RbxSession {
@@ -47,13 +47,16 @@ impl RbxSession {
     }
 
     fn path_created_or_updated(&mut self, path: &Path) {
-        if let Some(instance_id) = self.paths_to_node_ids.get(path) {
-            // TODO: Replace instance with ID `instance_id` with new instance
-        }
+        let imfs = self.imfs.lock().unwrap();
+        let root_path = imfs.get_root_for_path(path)
+            .expect("Path was outside in-memory filesystem roots");
 
-        // TODO: Crawl up path to find first node represented in the tree or a
-        // sync point root. That path immediately before we find an existing
-        // node should be read into the tree.
+        let closest_path = self.paths_to_node_ids.descend(root_path, path);
+        let &instance_id = self.paths_to_node_ids.get(&closest_path).unwrap();
+
+        let snapshot = snapshot_instances_from_imfs(&imfs, &closest_path)
+            .expect("Could not generate instance snapshot");
+        reconcile_subtree(&mut self.tree, instance_id, &snapshot);
     }
 
     pub fn path_created(&mut self, path: &Path) {
@@ -167,7 +170,7 @@ fn construct_instance_node<'a>(
         name: Cow::Borrowed(instance_name),
         properties: HashMap::new(),
         children,
-        update_trigger_paths: Vec::new(),
+        source_path: None,
     }
 }
 
@@ -225,7 +228,7 @@ fn snapshot_instances_from_imfs<'a>(imfs: &'a Imfs, imfs_path: &Path) -> Option<
                 class_name: Cow::Borrowed(class_name),
                 properties,
                 children: Vec::new(),
-                update_trigger_paths: vec![file.path.clone()],
+                source_path: Some(file.path.clone()),
             })
         },
         ImfsItem::Directory(directory) => {
@@ -239,7 +242,7 @@ fn snapshot_instances_from_imfs<'a>(imfs: &'a Imfs, imfs_path: &Path) -> Option<
                     name: Cow::Borrowed(""), // Assigned later in the method
                     properties: HashMap::new(),
                     children: Vec::new(),
-                    update_trigger_paths: vec![directory.path.clone()],
+                    source_path: Some(directory.path.clone()),
                 }
             };
 
diff --git a/server/src/rbx_snapshot.rs b/server/src/rbx_snapshot.rs
index b8b3f32f..c7dd7b23 100644
--- a/server/src/rbx_snapshot.rs
+++ b/server/src/rbx_snapshot.rs
@@ -12,7 +12,7 @@ pub struct RbxSnapshotInstance<'a> {
     pub class_name: Cow<'a, str>,
     pub properties: HashMap<String, RbxValue>,
     pub children: Vec<RbxSnapshotInstance<'a>>,
-    pub update_trigger_paths: Vec<PathBuf>,
+    pub source_path: Option<PathBuf>,
 }
 
 fn reify_core(snapshot: &RbxSnapshotInstance) -> RbxInstance {