private function convertPrincipal($principalUri, $toV2 = null) {
        if ($this->principalBackend->getPrincipalPrefix() === 'principals') {
            list(, $name) = \Sabre\Uri\split($principalUri);
            $toV2 = $toV2 === null ? !$this->legacyMode : $toV2;
            if ($toV2) {