[Windows Phone 7 Development and Hacking] wheresmybeaver:
The first thing you should do is make an update of your phone through Zune; this way you can keep a backup of your Chevron-unlocked NoDo state for rollback in case any of the updates don't work as planned. A backup happens automatically when you start the Zune update process, or you can force it using the backup tool on this forum. In either case, once the backup is done, make a backup of that in another folder! You can find the backup at C:\Users\<username>\AppData\Local\Microsoft\Windows Phone Update

Once that's done, you can use the U2M7720_V1.2 XAP provided by ansar (first post of this thread) and follow his instructions, which essentially involve:

1. Download and install Connection Setup (the HTC app) from the Marketplace

2. Run the U2M7720 app, tap on U2B2 and finally tap on done

3. Go to the language tab and tap on the language you want to add and finally tap on done

4. Tap on Chevron Unlock (this will exit U2M7720 - that's expected)

5. DO NOT run Connection Setup now

6. Go to Zune on your Windows PC, go to settings and the Update option

7. An update will be found, so go ahead and update (if at this stage a backup is made, be sure to take the opportunity to backup that folder I mentioned previously when it's done).

8. IF your update ends up being a two-step update (ie the phone reboots at some stage to the usual Start screen with the tiles, and it hasn't been updated to Mango yet), then you will need to pull the USB plug on the update process once the tile screen appears on your phoneand proceed to the next step. If you have been updated to Mango in one step, skip to 13

9. Before reconnecting to your PC / syncing with Zune, launch the Connection Setup app and apply the settings.

10. Run the U2M7720 app again and tap on Chevron Unlock again (in case your phone relocks again in future).

10.b. Optionally, check a sideloaded app on your phone to make sure it's still running. If so, continue the steps.

11. Reconnect your phone to your PC, run Zune, go back to Settings again and click Update. An update will be found, so continue to Update

12. When the update is completed (this one will take a while, don't disconnect), you will find that you are on Mango. Disconnect your phone.

13. Run Connection Setup and apply the settings.

14. Run the U2M7720 app again and tap on Chevron Unlock again (in case your phone relocks again in future).

15. Check your sideloaded apps to make sure they're still OK.

You can restore to your NoDo image at any time by going into Zune on your PC, click on Settings, and hitting Restore in the Update page.

These are exactly the steps I used to update my O2 UK branded HTC HD7 from NoDo to Mango, keeping Chevron unlock intact. The only caveat I've noticed so far is that while Mango is deinfitely installed, I don't seem to have gotten the HTC firmware update that optimizes Mango for my handset (so, for example, battery life may be hampered and I can't get the compass to work with Mango apps). I expect this update will arrive separately at a later date.

Many thanks to ansar for his development of the XAP tool and his assistance with all of this. Let me know if there are any steps I've missed or gotten wrong.