By the time the single currency becomes a reality, the EU will speak at least 11 different languages. Banknotes have to be written in a language. The idea of each note being written in 11 separate languages is silly; there would not be enough room. Then, of course, there is the insurmountable problem of choosing designs which would be acceptable and meaningful to all members.
When the time comes the problem will not be how to persuade countries to ditch their historic denominations, but rather of dissuading the regions from reviving their own.