Your registered trademarks in U.S. are not applicable in China. Therefore, if you see good opportunities in China and are looking to remain in the market for the medium to long-term, then it is important to have your IP taken care of right at the beginning. China is a first-to-file country meaning that, all else being equal, whoever applies for a trademark first, will obtain the registration. Unless you can manage to do business without your existing brand, seriously consider registering your trademark in China.
For a foreign company that does not have a base in China, a trademark agent or lawyer is required to register. The cost of registering a trademark using an agent is around $500 for one product class. A straight forward trademark registration can take approximately 18 months.
When registering your trademark you will want to make sure your company registers trademarks in all the appropriate product classes to ensure proper protection. For example, it may be obvious to apply for your trademark under "food product" but you may also want to consider registering your trademark under "health food product" and "beverage" and "processed foods" - these are just fictitious examples to give you an idea. Please refer to the Nice Classification for the actual classes of good used in China. The important point is to cover all the classes of goods you are interested in (and even not interested in) so that you avoid someone registering your trademark in another class of goods and then competing with your product. Clearly, it is cost prohibitive for the average company to cover all product classes. This is when a trademark agent or lawyer may be able to provide some expert advice.
For your trademark registration, you will want to have Chinese versions of your trademarks. This is important because Chinese consumers will need a Chinese name in order to identify your product. A good Chinese trademark is just as important as your English and/or French one. Look for assistance from a person with a strong grasp of Mandarin Chinese to come up with a Chinese trademark so that you avoid any inappropriate or poor trademarks. Treat the registration of your Chinese trademark as important as your English/French trademark - you will want to arrange the registration yourself as opposed to, for example, letting your distributor or local partner register for you. It is also important to ensure that your trademark will not infringe on any existing registered trademarks in China. Your trademark agent/lawyer can conduct a search to minimize this risk.
In terms of prevention, as in U.S. or any other market in the world, there is no guarantee that another party will not infringe your rights. Without a registered trademark, however, there would be no effective remedies to put an end to the infringing activities. A registered trademark ensures that you have legal recourse. Admittedly, the last thing any company wants to deal with is a costly court case or lawyer fees, so prevention is key to ensure that problems do not get out of hand. It is important to monitor your partners and distributors as well as the marketplace to ensure that your trademark is not being infringed and that no one is registering copycat trademarks. It is also important to be selective with your distributors which means proper due diligence to ensure they have good reputations. Finally, even after selecting a good partner, it may be prudent to insert intellectual property-related clauses in your agreements.