I assume that you mean changes in interpretation rather than the formal changes in the amendments. Either way, yes, the Constitution is still relevant, and remains the point of reference for our courts as well as the basic framework for our government. The recent Supreme Court case on the Affordable Healthcare Act is an example. It hinged on a particular interpretation of the Commerce as well as the Taxation clause in the Constitution. Another example would be the Electoral College, which, while straying far from its original intent in that electors are essentially "rubber stamp" voters, will still decide our election no matter who wins the popular vote. You can say that we interpret the Constitution quite differently today, but not really that it is no longer relevant.
It's hard to say that we have had too many changes to the Constitution given that there have only been 27 and 10 of those were enacted essentially right away.
The Constitution is still absolutely relevant. We can see that from the fact that people of all political stripes refer to it constantly to support their positions.
If I were you I would focus on the flexibility of the Constitution to respond to our changing needs. Would you really want to live in a world governed by 18th century laws, or laws that cannot change? The Constitution is often described as a “living document” because it can change. There is no way the Founding Fathers could have conceived of smart phones or the internet. If we want the Constitution to be relevant, it has to be able to change with the times.
This is for an essay I have to write...your comments are much appriciated. :)