I can not give you an answer to your question, but whenever I had problems with kefir, I was told to thoroughly rinse the tuber and restart the kefir.
On this German Website they recommend to restart with 200ml of milk and 1 tablespoon of lactose for a recovery of the kefir. I do not know if this could change the yeast to bacteria ratio.
On a sideline, this Russian website recommend a ratio of 107 CFU/g of lactic acid bacteria to 104 CFU/g of yeast for health reasons. If you don't get the yeast to bacteria ratio down, maybe a fresh start with a kefir with 104:107 could help?
The dynamics of the Grain growth are not currently well understood, much less its formation. Some researchers attempted to create grains in laboratory, but failed.
There are over thirty or fourty strains of microbes in the kefir, and their ecological relationships are very complex. One could call the whole kefir culture an ecosystem of its own.
The grains grow either because the matrixes of existing grains entice the microbes to create more grain matrix, or because there are specialized microorganisms inside the grain that are responsible for creating more grain.
What is known is that the grains need Kefiran to grow, and Kefiran needs Alcohol in its synthesis. Commercial cultures of kefir often omit the alcohol-creating yeast for obvious reasons, so it too adds to the impossibility of getting Kefir grains from commercial kefir.
Either way, the fact is there grains do not come into being on their own. New grains are split from existing grains.
There is a small possibility that a kefir culture from grains that was strained could form new grains, but its not confirmed.
The commercial cultures are sold with around five to ten strains, much less than full blown kefir grains. They won't form grains, since the strains were hand-picked in lab by their function, and probably the grain-forming ones were left out.
Best Answer
It sounds like the kefir was further fermented by a second, more aggressive stain of acetic or lactic acid bacteria introduced after opening. It's likely safe, but probably not very tasty. And as with any unexpected microbial activity: When in doubt, throw it out.