Most 401k plans have a number of different options for investors. While you can't necessarily "freeze" a 401k account, most of them do offer money market (or, cash equivalent) alternative. Ask your plan administrator for specifics.
Over the long-term, however, diversified portfolios do tend to provide the best risk/return characteristics.
I'm not hearing much about the Yuan replacing the dollar, except for certain foreign transactions. I've written more about dollar concerns here: http://www.stratfi.com/blog/69-what-comes-after-the-dollar.html
Betty, I would be careful trying to "predict" events in the market. I've never seen anyone who can do it consistently over time and it usually leads to more harm than good. That being said, if you need/want to make a change in your 401K, it is very likely that you could move to a fund within your 401K that would be either a money market or a very short term fixed income position. It is a matter of looking at your investment choices within the plan. I don't believe a "freeze" is possible though. One last note, more money has been lost trying to "predict" events in the market than virtually any other common mistake. If you are contributing to your 401K and the market goes down you are simply buying those securities cheaper (on sale). Over a long period of time you benefit from market corrections. Good luck.
No one can predict the future. Even the Wall Street bullies. They have a vested interest in keeping you trading. It generates fees for them and the broker. Instead build a prudent globally diversified portfolio at your risk level and remain disciplined. It also helps to rebalance periodically.