lib/Dropbox/OAuth1Upgrader.php
function createOAuth2AccessToken($oauth1AccessToken)
    {
        OAuth1AccessToken::checkArg("oauth1AccessToken", $oauth1AccessToken);

        $response = self::doPost($oauth1AccessToken, "1/oauth2/token_from_oauth1");

        if ($response->statusCode !== 200) throw RequestUtil::unexpectedStatus($response);

        $parts = RequestUtil::parseResponseJson($response->body);

        if (!array_key_exists('token_type', $parts) || !is_string($parts['token_type'])) {
            throw new Exception_BadResponse("Missing \"token_type\" field.");
        }
        $tokenType = $parts['token_type'];
        if (!array_key_exists('access_token', $parts) || !is_string($parts['access_token'])) {
            throw new Exception_BadResponse("Missing \"access_token\" field.");
        }
        $accessToken = $parts['access_token'];

        if ($tokenType !== "Bearer" && $tokenType !== "bearer") {
            throw new Exception_BadResponse("Unknown \"token_type\"; expecting \"Bearer\", got  "
                . Util::q($tokenType));
        }

        return $accessToken;
    }
function createOAuth2AccessToken($oauth1AccessToken)
    {
        OAuth1AccessToken::checkArg("oauth1AccessToken", $oauth1AccessToken);

        $response = self::doPost($oauth1AccessToken, "1/oauth2/token_from_oauth1");

        if ($response->statusCode !== 200) throw RequestUtil::unexpectedStatus($response);

        $parts = RequestUtil::parseResponseJson($response->body);

        if (!array_key_exists('token_type', $parts) || !is_string($parts['token_type'])) {
            throw new Exception_BadResponse("Missing \"token_type\" field.");
        }
        $tokenType = $parts['token_type'];
        if (!array_key_exists('access_token', $parts) || !is_string($parts['access_token'])) {
            throw new Exception_BadResponse("Missing \"access_token\" field.");
        }
        $accessToken = $parts['access_token'];

        if ($tokenType !== "Bearer" && $tokenType !== "bearer") {
            throw new Exception_BadResponse("Unknown \"token_type\"; expecting \"Bearer\", got  "
                . Util::q($tokenType));
        }

        return $accessToken;
    }
Given an existing active OAuth 1 access token, make a Dropbox API call to get a new OAuth 2
access token that represents the same user and app.

See <a href="https://www.dropbox.com/developers/core/docs#oa1-from-oa1">/oauth2/token_from_oauth1</a>.

@param OAuth1AccessToken $oauth1AccessToken

@return string
The OAuth 2 access token.

@throws Exception