I don't know if it's as simple as that. I went from FB17 to FB21 by ...
1) Booting into Rogue FB17 CWM repack recovery
2) Installing Calk's wipe all ZIP
3) Installing FB21 ROM
4) Installing FB21 Modem
All went fine. No brick.
I later used sfhub's autoroot tool to push over the FB21 CWM kernel repack. Again, rebooted fine.
EDIT - I've also Nandroid restored my GB ROM to get a GPS lock, shutdown, booted back into CWM and restored my FB21 ICS ROM. Don't know what to say really, but it seems I've done a lot of the things that people are suspecting are causing the bricks.
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