diff --git a/fluent-package/apt/systemd-test/downgrade-to-v4.sh b/fluent-package/apt/systemd-test/downgrade-to-v4.sh
index 915675771..8d5d10cfd 100755
--- a/fluent-package/apt/systemd-test/downgrade-to-v4.sh
+++ b/fluent-package/apt/systemd-test/downgrade-to-v4.sh
@@ -1,5 +1,14 @@
 #!/bin/bash
 
+function debug_service_files() {
+    echo $1
+    set +e
+    ls -la /usr/lib/systemd/system/ |grep -E 'td-agent|fluent-package'
+    ls -la /etc/systemd/system
+    ls -la /usr/lib/systemd/system/multi-user.target.wants/
+    set -e
+}
+
 set -exu
 
 . $(dirname $0)/../commonvar.sh
@@ -20,14 +29,21 @@ sudo apt install -V -y \
     /host/${distribution}/pool/${code_name}/${channel}/*/*/fluent-package_*_${architecture}.deb \
     /host/${distribution}/pool/${code_name}/${channel}/*/*/td-agent_*_all.deb
 
-# td-agent.service is already masked (link to /dev/null), and remove td-agent.service alias not to conflict with v4
-sudo systemctl unmask td-agent
+sudo systemctl is-active fluentd
+sudo systemctl is-active td-agent
+sudo systemctl stop fluentd
 
 # Even though removing fluent-package, log and .conf are kept. dpkg reports "rc fluent-package" and "rc td-agent" status.
-sudo apt remove -y fluent-package
+sudo apt remove -y fluent-package td-agent
+
+debug_service_files "DEBUG: after remove fluent-package td-agent"
 
 # fluentd.service is already masked (link to /dev/null), then remove it.
 sudo systemctl unmask fluentd
+# purge inacive (dead) symbolic links
+sudo systemctl disable td-agent
+
+debug_service_files "DEBUG: after unmask fluentd"
 
 # Drop symbolic links and recreate real directory.
 sudo rm -f /var/log/td-agent
@@ -45,6 +61,11 @@ sudo chown td-agent:td-agent /etc/td-agent
 sudo mv /etc/fluent/td-agent.conf /etc/td-agent/
 curl -fsSL https://toolbelt.treasuredata.com/sh/install-${distribution}-${code_name}-td-agent4.sh | sh
 
+debug_service_files "DEBUG: after downgrade"
+
+# re-create symlink to point service file again
+sudo systemctl enable td-agent --now
+
 # Test: service status
 (! systemctl status --no-pager fluentd)
 (systemctl status --no-pager td-agent)