If you plan on keeping drinking kefir just get yourself some real grains. Trust me you will enjoy that kefir a lot more. The real kefir culture in the grains is so strong that scientist that deliberatly inoculated raw milk with E Coli., strep. c, and other bad bacteria found that makin kefir using grains and the contaminated milk was no problem. The kefir culture completely removed all bad bacteria, they could not be traced back after 24hr fermentation.
Just using the sour milk as a starter is a very unstable proces. It is impossible to tell what bacterial culture might arise when working this way. Be safe get some grains.