Well, it's a good thing you didn't go out and buy the wrong filter media ^^ You should probably just rinse off the filter media in a bucket of tank water (unless it is really bad). I replce my media every now and then because it comes in two parts and bacteria grows on both - since you only have one major piece of filter media, you may not want to get a completely new one.
If it gets really ratty, leave the old media in the tank for awhile whne you change it, like you originally planned to do.
A week should suffice since it's not a new tank; there is bacteria in the gravel to help it along as well.