If they do that, all hell will break loose. It's totally unprecedented and constitutionally in all American History, this is not legal. Unilaterally, this move will be invalid.
Tiagong that the Trump Administration had asked Japan to convert their bonds to long term notes but Japan had rejected the 100-year bond proposal from the U.S. and left without making any deal last week.
The Trump administration had offered security guarantees to Europe and Japan. In return, those countries would be expected to upgrade their bonds to 100-year terms. But Japan apparently didn’t agree and left the U.S. without finalizing a trade deal, despite coming with that intention.
Now, if a country like Japan, which is a close ally of the US will not accept a 100-year bond, then it’s highly unlikely that Europe will. No sensible country will want to own long-term U.S. bonds because they are illiquid.