I haven't built or released any update.zips before so I might be doing something wrong. But "it works for me". I built a FRF72 from FRF50 rooted update. It's a lot bigger than Google's because it's full files rather than patched files, but it installs fine from Amon_RA onto a FRF50 install and upgrades it to FRF72. And your root will not be lost.
Do a nandroid backup before flashing! I'm not responsible for any problems caused.
Wipe is not required.
This update should be able to apply to any Nexus One FRF50 build, but I only tested against Paul's no-preq update. Wipe is not required. Do a nandroid backup before flashing. I'm not responsible for any problems or data loss.
FRF72_from_FRF50_rooted_KB_20100619-signed.zip 40MB. MD5: e52c58021ffa31e8e87d1e9e7d7fdb66
Feel free to repost the zip. I'm on dreamhost, dunno how much traffic this will generate or how they will handle it.