Can I download Ubuntu desktop on Lubuntu?

You won't end up exactly the same result; but I'd just

apt purge lubuntu-desktop
apt install ubuntu-desktop

Lubuntu doesn't come with any OEM kernel options, 3rd party drivers etc so you won't get any of those (they help make the Ubuntu Desktop ISO much larger); and you'll still end up using a large % of the bandwidth the Ubuntu ISO requires (ISOs are compressed; you're missing the stuff that Lubuntu doesn't provide; OEM kernels, 3rd party graphics drivers etc; though some of those can be added later too - meaning download saved is even less)

I'd consider starting with an existing ISO if you have a like one (even if it's a release or two old; or close release but different flavor) & just download the differences using zsync. This option would be what I'd consider; and regularly use myself (using other flavors or prior releases..)