I've rekeyed my old BMW locks, it's very easy. I'm just wanting to replace the inevitable worn bits inside the 70s mechanism from 600k of hard use that a simple rekey won't fix. But it's not a huge priority. I'll keep digging.
What I would do is buy new individual cylinders and then re-key the doors to match the new ignition cylinder. The door cylinders are way easier to do and the process is not a destructive as disassembling the ignition cylinder.