No need to cut it open.
I fished out the old filter and the charcoal then dumped out by itself. You can turn it upside down, dump the new charcoal in through the grate. Then roll up the filter and wiggle it through the largest grate slot you have. Once it is inside, use a couple metal picks or probes to unfold it into place. Then turn it right side up and the charcoal will hold the filter in place, nicely.
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