diff --git a/scripts/build_env/appregdef_render.py b/scripts/build_env/appregdef_render.py
index 7f4551c70..fc8452c17 100644
--- a/scripts/build_env/appregdef_render.py
+++ b/scripts/build_env/appregdef_render.py
@@ -4,6 +4,15 @@
 from env_template.process_env_template import process_env_template
 from render_config_env import EnvGenerator
 
+def copy_app_reg_defs(render_dir, env_dir):
+    for dir_name in ["AppDefs", "RegDefs"]:
+        src = Path(render_dir) / dir_name
+        dst = Path(env_dir) / dir_name
+
+        if src.exists():
+            shutil.copytree(src, dst, dirs_exist_ok=True)
+
+
 if __name__ == '__main__':
     template_version = process_env_template()
 
@@ -31,13 +40,6 @@
     render_context = EnvGenerator()
     render_context.process_app_reg_defs(env_name, render_context_vars)
 
-    for dir_name in ["AppDefs", "RegDefs"]:
-        src = Path(render_dir) / dir_name
-        dst = Path(env_dir) / dir_name
-
-        if dst.exists():
-            shutil.rmtree(dst)
-        if src.exists():
-            shutil.move(src, dst)
+    copy_app_reg_defs(render_dir, env_dir)    
 
     update_generated_versions(env_dir, BUILD_ENV_TAG, template_version[NamespaceRole.COMMON])
diff --git a/scripts/build_env/tests/app_reg_defs/test_appregdefs.py b/scripts/build_env/tests/app_reg_defs/test_appregdefs.py
index e3df35a57..2f086eb11 100644
--- a/scripts/build_env/tests/app_reg_defs/test_appregdefs.py
+++ b/scripts/build_env/tests/app_reg_defs/test_appregdefs.py
@@ -8,6 +8,7 @@
 from render_config_env import EnvGenerator
 from envgenehelper.test_helpers import TestHelpers
 from envgenehelper.business_helper import NamespaceRole
+from scripts.build_env.appregdef_render import copy_app_reg_defs
 
 
 class TestAppRegDefRendering:
@@ -106,3 +107,24 @@ def test_negative_appregdef_rendering(self, test_number, expected_exception):
         with pytest.raises(expected_exception):
             render_context.process_app_reg_defs(self.env_name, context_vars)
 
+    def test_appregdefs_merge_behavior(self, request):
+        self._setup_render_dir()
+
+        render_context = EnvGenerator()
+        context_vars = self._get_render_context("TC-001-001")
+
+        render_context.process_app_reg_defs(self.env_name, context_vars)
+
+        render_dir = Path(context_vars["current_env_dir"])
+        env_dir = Path(f"/tmp/environments/{self.cluster_name}/{self.env_name}")            
+        appdefs_dir = Path(f"{env_dir}/AppDefs")
+        appdefs_dir.mkdir(parents=True, exist_ok=True)
+        (appdefs_dir / "existing.yml").write_text("keep-me")       
+
+        copy_app_reg_defs(render_dir, env_dir)
+
+        assert (appdefs_dir / "application-1.yml").exists()
+        assert (appdefs_dir / "existing.yml").exists()
+
+        shutil.rmtree(appdefs_dir, ignore_errors=True)
+        shutil.rmtree(render_dir / "AppDefs", ignore_errors=True)
\ No newline at end of file