The in-key ID chip that electronically tells the car 'OK to go' is the biggie. They cost. Simply replacing door and trunk cylinders (keyed alike) shouldn't be prohibitive, depending how thoroughly they're integrated with the powerlocks. If the chip's anti-theft protection starts at the door you're screwed. Back when I did it, Honda sold individual cylinders or complete sets for $20-30 per. The hardest part of the change was getting the inner door panels off. The cylinders are made to be a 2 second install on the line.

Then you carry an ignition key and a door key. And just in case the guy with my keys came back on a night I hadn't locked up, I did put a kill switch under the glove box.