The old and the new 2

When did the 50p become a different size? I'm guessing it was some time between 1973 and 2006 (the dates of these two) but I don't remember it. I took the one on the left in the charity shop where I work only to have it refused by the bank at the end of the day.
I wonder how many hands the old one has passed through in it's 42 years.