I'd stick with your current plan and add a minor. By having a journalism degree you're not locking yourself into journalism, and you don't need a degree in theatre to do theatre. Following that, just because someone has a degree in theatre doesn't mean they're guaranteed to do theatre (by their own choice or, more often, a lack of paying work).
It's quite a romantic notion to sit in the audience of a show and watch the curtain go up or be regaled by friends' war stories of how the show went on despite the longest of odds, but if you've got no experience I would second Matthew's question of how do you know this is what you want?
I can speak from a bit of experience on your situation - I didn't go in to school with Stage Management as my declared major. It wasn't until after I did a professional internship that I realized I wanted to stage manage for the rest of my life. I was good at it, and I loved doing it. I was certain it was the career for me. I switched into the SM program, completed it, and worked professionally. That internship was 11 years ago this summer, and guess what - I'm no longer stage managing.
Can you handle long hours for extremely low pay - possibly into your 30s?
Can you handle sacrificing your nights/weeks consistently and everything that goes with it? Your social life is wildly different from anyone outside of theatre due to the hours, relationships with folks in 9-5 industries are challenging, you'll miss holidays with loved ones, important family events, weddings, funerals, etc...
Can you handle not being appreciated for the work you do?
The list goes on...
You've changed your mind once, it's likely you will again - maybe not tomorrow, maybe not next year - but it's likely you will. That's not to say don't follow your dreams or don't pursue your passions, but proceed with a bit of caution. We must crawl before we can walk. Get your feet wet and see if it's really for you.