The hardest part was getting the trigger assembly back in....
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There was no reason to take it out. In fact, you don't need to disassemble anything else to replace the safety.
1. Verify the shotgun is unloaded, including the chamber.
2. Cock the shotgun and do NOT pull the trigger.
3. Remove the old screw (some are hard, others are easy), safety button, detent plate, & detent ball. Take care not to lose the ball or the spring under it.
4. Use a dab of grease to hold the ball in position on top of the spring.
5. Put the plate in place, making sure the center hole lines up with the screw hole.
6. Put the new safety in place and tighten the screw.
7. Verify proper function.
It took longer to type that that it did to replace the one on my 500.
Note, If your old screw is hard to get out, you can grab the plastic safety with a pair of pliers and break it. It will break right at the screw hole, then you can remove the screw with pliers, if necessary.