Confused by runeforging. Is there a way to upgrade a unique twice?

Can I runeforge something twice?
E.g., I forged a low level unique to have it become a level 50-ish item, but now i'm level 78, and it wont let me put it into the anvil's left slot.

Also can we runeforge high level uniques?

If the answer is yes to either of these, please tell me how.