Converting copper to gold can be done legitimately - all it takes is a level 3 furnace, 100 copper ingots, and a portal!
Step 1: Craft your 100 copper ingots into copper coins. You should have 1000 copper coins.
Step 2: Go to your portal, and exchange the 1000 copper coins for 10 gold coins.
Step 3: Go back to your furnace, and craft the 10 gold coins into one gold ingot.
Step 4: TASK ACCOMPLISHED! Yes, I know that it takes a bunch of copper - but for me, copper is the most common ore in the world, so I have a bunch. And, as we know, copper becomes useless in more advanced worlds.
The same is possible when done oppositely - you can craft 1 gold ingot into 100 copper ingots.
Step 1: Craft your gold ingot into 10 gold coins at your level 3 furnace.
Step 2: Go to your portal, and exchange your 10 gold coins for 1000 copper coins.
Step 3: Return to your furnace, and craft your 1000 copper coins into 100 copper ingots.
Step 4: Enjoy your ingots!
Hope this helped guys!