diff --git a/app/controllers/manager/users_controller.rb b/app/controllers/manager/users_controller.rb
index 5f1e52ad5..2586c5a51 100644
--- a/app/controllers/manager/users_controller.rb
+++ b/app/controllers/manager/users_controller.rb
@@ -2,14 +2,25 @@ module Manager
   class UsersController < Manager::ApplicationController
     def update
       user = User.find(params[:id])
-      new_email = params[:user][:email]
-      user.skip_reconfirmation!
-      user.update(email: new_email)
-      if (user.valid?)
-        flash[:notice] = "L'email a été modifié en « #{new_email} » sans notification ni validation par email."
+
+      preexisting_user = User.find_by(email: targeted_email)
+
+      if preexisting_user.nil?
+        user.skip_reconfirmation!
+        user.update(email: targeted_email)
+
+        if (user.valid?)
+          flash[:notice] = "L'email a été modifié en « #{targeted_email} » sans notification ni validation par email."
+        else
+          flash[:error] = user.errors.full_messages.to_sentence
+        end
       else
-        flash[:error] = "« #{new_email} » n’est pas une adresse valide."
+        user.dossiers.update_all(user_id: preexisting_user.id)
+
+        user.instructeur&.update(user: preexisting_user)
+        user.expert&.update(user: preexisting_user)
       end
+
       redirect_to edit_manager_user_path(user)
     end
 
@@ -72,5 +83,11 @@ module Manager
       end
       redirect_to emails_manager_user_path(@user)
     end
+
+    private
+
+    def targeted_email
+      params[:user][:email]
+    end
   end
 end
diff --git a/spec/controllers/manager/users_controller_spec.rb b/spec/controllers/manager/users_controller_spec.rb
index 2e7ea86ce..4649b5d99 100644
--- a/spec/controllers/manager/users_controller_spec.rb
+++ b/spec/controllers/manager/users_controller_spec.rb
@@ -21,24 +21,54 @@ describe Manager::UsersController, type: :controller do
 
     subject { patch :update, params: { id: user.id, user: { email: nouvel_email } } }
 
-    describe 'with a valid email' do
-      let(:nouvel_email) { 'nouvel.email@domaine.fr' }
+    context 'when the targeted email does not exist' do
+      describe 'with a valid email' do
+        let(:nouvel_email) { 'nouvel.email@domaine.fr' }
 
-      it 'updates the user email' do
-        subject
+        it 'updates the user email' do
+          subject
 
-        expect(User.find_by(id: user.id).email).to eq(nouvel_email)
+          expect(User.find_by(id: user.id).email).to eq(nouvel_email)
+        end
+      end
+
+      describe 'with an invalid email' do
+        let(:nouvel_email) { 'plop' }
+
+        it 'does not update the user email' do
+          subject
+
+          expect(User.find_by(id: user.id).email).not_to eq(nouvel_email)
+          expect(flash[:error]).to match("Courriel invalide")
+        end
       end
     end
 
-    describe 'with an invalid email' do
-      let(:nouvel_email) { 'plop' }
+    context 'when the targeted email exists' do
+      let(:preexisting_user) { create(:user, email: 'email.existant@domaine.fr') }
+      let(:nouvel_email) { preexisting_user.email }
 
-      it 'does not update the user email' do
-        subject
+      context 'and the old account has a dossier' do
+        let!(:dossier) { create(:dossier, user: user) }
 
-        expect(User.find_by(id: user.id).email).not_to eq(nouvel_email)
-        expect(flash[:error]).to match("« #{nouvel_email} » n’est pas une adresse valide.")
+        it 'transfers the dossier' do
+          subject
+
+          expect(preexisting_user.dossiers).to match([dossier])
+        end
+      end
+
+      context 'and the old account belongs to an instructeur and expert' do
+        let!(:instructeur) { create(:instructeur, user: user) }
+        let!(:expert) { create(:expert, user: user) }
+
+        it 'transfers instructeur account' do
+          subject
+          preexisting_user.reload
+
+          expect(preexisting_user.instructeur).to match(instructeur)
+          expect(preexisting_user.expert).to match(expert)
+        end
       end
     end
   end