As Dirk and Ghanima stated in their answers, the service may be overwritten by future updates/upgrades of the system. But systemd has a mechanism to avoid this. You can use a drop in file. An update/upgrade will only modify the main service. The drop in file is not effected by it and will also modify the updated main service. You can simply create a drop in file with:
rpi ~$ sudo systemctl edit raspberrypi-net-mods.service
In the empty editor insert these statements, save them and quit the editor:
[Service]
ExecStart=
ExecStart=/bin/cp /boot/wpa_supplicant.conf /etc/wpa_supplicant/wpa_supplicant.conf
The first empty ExecStart=
is important. It will suppress the original ExecStart in the main service.
Check the source of the whole service with:
rpi ~$ systemctl cat raspberrypi-net-mods.service
You will find the drop in at the end.