best solution for CEL P0300?
 

My car is just over 90k miles, and recently I've been getting the following codes:
P0300 Misfire Damaging to Catalytic Converter
P0301 Misfire, Cylinder 1, Damaging to Catalytic Converter
P0303 Misfire, Cylinder 3, Damaging to Catalytic Converter

I believe this suggests an issue with the coils. I have a pretty complete history for the car, and don't believe the coils have been changed before. It is worth the time/effort of swapping them around to see if they're bad, or should I just go ahead and replace them? If replacing, should I do all six at once, or just the two that are reporting errors? One challenge is that I don't have a code reader, so I need to get them read somewhere to see what any changes cause.

Seems it's been a while for the plugs, so I'll likely change all of those as well.

I would swap the coils to make sure thats the problem. Could be as simple as plugs or it could also be injectors.