You'll need to cp /etc/systemd/system/bluetooth.target.wants/bluetooth.service /etc/systemd/system/ so you can override the command line to add --compat.
# cat /etc/systemd/system/bluetooth.service
[Unit]
Description=Bluetooth service
Documentation=man:bluetoothd(8)
[Service]
Type=dbus
BusName=org.bluez
ExecStart=/usr/lib/bluez5/bluetooth/bluetoothd --compat
NotifyAccess=main
#WatchdogSec=10
#Restart=on-failure
CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_BIND_SERVICE
LimitNPROC=1
[Install]
WantedBy=bluetooth.target
Alias=dbus-org.bluez.service