OpenBSD 5.5 will be year 2038 ready, but this requires a change to a 64 bit time type. This results in a "flag day" event, where old binaries will not run on the new kernel, and the new binaries won't run on the old kernel, and some file formats will be changing. A remote, no-console process will be provided, but it will be a more touchy update process than usual.

It is also pretty good practice for anytime those servers go bad. It helps to be able to put temporary replacements in service from whatever I have lying around. I can save the hot spares for machines that have user data on them (e-mail, file servers).

I.E. The default approach, incremental upgrade, is the only way to ensure your OpenBSD system doesn't fail.

I look at it this way, if all I'm really doing is adding some flags or configuration files, I would rather just blow it away and do the reinstall. Last couple of times I did that with my firewall, it was a 20 minute install.