Phalcon 2 and Phalcon 1.3 have the same functionality, you can upgrade from 1.3 to 2.0 without breaking your application.
Having tested my application on both, i have some changes to do to make it works :
Phalcon\Config doesn't works the same exactly :
['key' => ['a', 'b', 'c']]
have not the same behavior in 1.3.2 and 2.0.0 : $config->key is a Phalcon\Config in 1.3.2 and an array in 2.0.0
the declaration of ResponseInterface changed :
2.0.0
Phalcon\Http\ResponseInterface::setExpires(DateTime $datetime)
1.3.2
Phalcon\Http\ResponseInterface::setExpires($datetime)
Phalcon\Di\InjectionAwareInterface::setDI changed also
2.0.0
Phalcon\Di\InjectionAwareInterface::setDI(Phalcon\DiInterface $di)
1.3.2
Phalcon\Di\InjectionAwareInterface::setDI($di)
...
It's not major changes and they are good from my point of view, but it's not an upgrade "without" breaking