I didn't have to replace any breakers. The load center and breakers were Eaton. They run about $5 for a 20 amp breaker.
I bought a
Reliance Controls Transfer Switch from Amazon.
You identify the circuits you want power with the generator.
This works well Then disconnect the wire from the circuit and tie it to the black wire going to the transfer(tx) switch. The corresponding red wire from the tx switch goes back into the breaker. The switch is a break-before-make that toggles from generator, off, and line. (Read that as "Not possible to shock a lineman or blow up your generator.") It took me about 6 hours total to identify circuits, calculate circuit load, balance load across power rails, watch YouTube videos, mount the outlet outside and the tx switch to the wall, and finally wire the load center.
It's a safe way to quickly power your house in an outage and perform regular testing. No hassle with exention cords, main breakers, or leaving your door or window open to run wires. Plus, when utility power is restored, the non-generator backed circuits energize normally. At your convienence, flip the switches back to line and power down the generator. You can also safely load test the generator with utility power present.