I'll see if there's anything of general use in my notes on migration. As nemo says, if you're running kenfs now, it's probably easiest to use cwfs(4) to keep using your existing file systems, with a small fossil for booting.
Migrating from venti to nventi is almost painless and maybe we can make it completely so. You'll likely want to at least add a 512MB bloom filter to improve performance, but nventi copes with venti arenas just fine. The only incompatibility I can think of is that nventi/venti doesn't accept all the options that venti/venti does, and refuses to start if you give it unknown options. It might be better to complain on stderr and in syslog and ignore venti options that nventi doesn't currently recognise.
