So, after I dug out my old netbook from the dust of my stash shelf, which was already equipped with lubuntu 14.04, I successfully installed ubuntu-device-flash utility via apt package manager, and used the utility to downgrade to 15.04.
Wish, this can be this easy on bionic (18.04).
I will look into the ubuntu-device-flash utility on ubports github and try to make it work.