YES. In 7 years with a company that got acquired by another and then eventually liquidated, my T&Cs changed about 5 times. I didn't have shares, but had a large profit share component which was fiddled with constantly. I think you should read Heidi Roizen's blog https://www.heidiroizen.com for some more examples.

Perhaps you should look at the big picture. Are your earnings above average? Was the stock grant the only reason you took the job? If you start legal action or make a lot of noise, your career will probably hit some roadblocks at that company and possibly others if the news gets out in the open.