Zheesh. This is silly on so many levels. First of all, you’re not really “trading” currency. More accurately you sell your greenbacks for the local currency (or another currency) at whatever rate the changer offers you. Rates vary from place to place, and in some countries, the black market rates (via private currency speculators) can be 5 to 10 times higher than official bank rates. In a country where currencies fluctuate very much or are experiencing rapid inflation, investment in hard currency (usu. Euros or Dollars) is a way to protect against these things.
Anyhow, I was in Europe in 1996 when one of the newer bills came out (I think it was the $20. Not the colored one like now, but the first new-style bill. It also might have been the $100, but I can’t remember for sure.) What did happen is that some banks (in England) were hesitant in changeing the bills because they had not been adequately educated on the new bill. This usually meant the teller would ask for a supervisor who would confirm that, yes, indeed the bill was genuine.
This is the only way I can see there be any problems in selling your funky new twenties. If you’re changing with some guys on the street, they might not be familiar with the new bill quite yet, so they won’t buy it, but most of your banks should be sufficiently forewarned.