As time passes, the previous design generally drops from circulation. When was the last time you saw a wheat penny? Similarly, it's pretty darn rare to find any coin higher than a nickel in denomination that is older than 1965 (1972 in the case of half-dollars) because that's the year they debased them from silver to junk metals.
What I've recommended to kids is that they start by saving and putting aside one nickel of every year and mint mark they can find. As time passes, and they find a better example of a given year and mint, they could replace it with the one in better shape, so their collection will, over time, become both more complete and of better quality. It's cheap and easy for them.
Ten years from now you'll start seeing the Jefferson/Monticello nickels become more and more rare. Frankly, though I realize updating the design is probably not entirely a bad idea, I still am saddened by it.
About two weeks ago. It's out of circulation now...
The change from silver to base metals meant that old coins were obviously worth more than face value. For other changes, though, I don't see it as such an issue. Eagle quarters are still in very wide circulation, and even the bicentenial ones show up from time to time. I would expect Monticello nickels to likewise remain around.