Uh, pretty sure a skill level 20 weapon will easily be an upgrade. This depends on many things, such as your overall atk, and how much weapon skills you already have though.
Sanahtlig has made a toolbox, one of which can calculate when on-element weapons with lower base attack are better.
For the record, for me, a skill level 1 weapon with 1600 Atk on-element weapon is already better than a 2100 Atk off-element. But this may not be true for you.While true, I disagree. Especially if you've found the element you want to main, leaving weapons un-upgraded because "I'll change them in the far future" is not a great plan. You'll get ahead with them now.