So you're asking if you can just replace the 8" woofers with 8" "sub"woofers? Yes, you can. Easiest way is to just drag the old speaker box with you to wherever you're buying the new ones and have them help. They can tell you what will work best.
There's several things you need to consider if you're going to upgrade a raw driver in a box. Driver size, Ohm load, crossover frequencies, response curves, etc. That's why it's best to just bring it in unless you know most of these factors.