Currently it is not possible to use the password modify extended operation to bind as one user and change another user's password without it being considered a password reset.
However it is possible to do this using the normal modify operation, as it supports the proxy auth control.
For example this changes user.14's password but does not reset it:
The closest we can get with the extended operation causes a reset of user.14's password:
It would be useful if the server's PasswordModifyExtendedOperation supported the proxyAuth control, perhaps with some constraints such as the proxy user having to be the same as the target entry.