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.

Please, also see this comment in a related issue https://github.com/ubports/ubuntu-touch/issues/381#issuecomment-431188052