Roll out the Old/Roll in the New!

Spent a moment comparing my first new £20 to an old one.  (My one observation is that it is very difficult to get the new one to lie down flat!)

You feel tempted to keep hold of what could be the last old one you see, or the first new one; but there isn't much point.  

I remember when £20 notes had 'William Shakespeare' on the reverse, which was a source of plenty of innuendo.