Why replace it? Cannondale's Headshok is an excellent shock! I have them on both my mountain bikes and my hybrid bike. It is a "monoshock" which is smoother than double stanchion shocks because there will be less side movement, so less chance for stiction.
One thing you should do is carefully check the boot around the shock. If it has any rips in it, it would probably be a good idea to have the shock re-lubed and the boot replaced - especially if you will be riding in rain/damp conditions.
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