Backup.
Rename old DLL to ..-OLD.DLL
Moved new DLL to .DLL
Reset

Turn Bluetooth on:
"BTTrayCE: THe Bluetooth Radio failed to turn ON due to insufficient driver memory available. You must perform a normal reset before you can turn ON the Bluetooth Radio."

Flight mode on.
Reset.
Turn bt on - same error.

Hard reset.
Apply 3500 CAB.
Reset.
Install Total Commander (file explorer won't show DLLs)
Rename old .OLD, move new into \windows
Turn bt on - works.

Restore contacts, calendar, email, so I can use the phone in an "interim no applications installed" mode to test bt, without losing contacts/conversations.

Will test bt for headset and DUN and report further.

Ehud