A Hard Reset restores the device to its default settings — the way it was when you first purchased it and turned it on. Any programs you installed, data you entered, and settings you customized on the device will be lost. Only Windows Mobile software and other pre-installed programs will remain.

There are 2 ways to perform a Hard Reset on this device:

Menu Path

1. Access the Settings Tab>All Settings>System>Clear Storage
2. Follow the onscreen instructions.

Keystroke Method

1. With the device powered off,  Press and hold the Talk/Send + End + Power keys until you see the message,
   • This operation will delete all your personal data, and reset all settings to manufacturer default. Press Vol Up to restore manufacturer default, or press other keys to cancel.
2. Release all buttons.
3. Press the Volume key up to perform the hard reset.

If a phone has been upgrade to Windows Mobile 6.5 (WM 6.5), will this cause the device to revert to WM 6.1?

No.......a hard reset just resets the current rom you have on your phone back to it's factory settings. The only way to change the rom is to re-flash the phone with a different rom.