Here's a way to remove the top grill from a P183.
First remove the top fan. Then remove the 4 screws and cylindrical black plastic washers (used to hold the gill in place tightly).
Then I used a flat putty blade to slowly pry the plastic top up (very slowly). Note: there are plastic tabs on the top cover along its length on both sides. The tabs on the left side (looking from back to front) are long tabs while the ones on the right are very small. Try to avoid breaking the long tabs.
What do you do with the grill removed? It's up to you. I plan to use the case without the top fan and want to seal up that opening. I thought I paint a piece of cardboard or use a piece of black vinyl flooring from home depot and wedging it into the grill. Then I'd put the grill back in.
Here are some photos.
Removing top grill from an Antec P183 (No cutting)
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