def change_ref(self, r0=None, lmax=None):
        """
        Return a new SHMagCoeffs class instance with a different reference r0.

        Usage
        -----
        clm = x.change_ref([r0, lmax])

        Returns
        -------
        clm : SHMagCoeffs class instance.

        Parameters
        ----------
        r0 : float, optional, default = self.r0
            The reference radius of the spherical harmonic coefficients.
        lmax : int, optional, default = self.lmax
            Maximum spherical harmonic degree to output.

        Description
        -----------
        This method returns a new class instance of the magnetic potential,
        but using a difference reference r0. When changing the reference
        radius r0, the spherical harmonic coefficients will be upward or
        downward continued under the assumption that the reference radius is
        exterior to the body.
        """
        if lmax is None:
            lmax = self.lmax

        clm = self.pad(lmax)

        if r0 is not None and r0 != self.r0:
            for l in _np.arange(lmax+1):
                clm.coeffs[:, l, :l+1] *= (self.r0 / r0)**(l+2)
                if self.errors is not None:
                    clm.errors[:, l, :l+1] *= (self.r0 / r0)**(l+2)
            clm.r0 = r0

        return clm
Return a new SHMagCoeffs class instance with a different reference r0.

        Usage
        -----
        clm = x.change_ref([r0, lmax])

        Returns
        -------
        clm : SHMagCoeffs class instance.

        Parameters
        ----------
        r0 : float, optional, default = self.r0
            The reference radius of the spherical harmonic coefficients.
        lmax : int, optional, default = self.lmax
            Maximum spherical harmonic degree to output.

        Description
        -----------
        This method returns a new class instance of the magnetic potential,
        but using a difference reference r0. When changing the reference
        radius r0, the spherical harmonic coefficients will be upward or
        downward continued under the assumption that the reference radius is
        exterior to the body.