When you export a TM from WorldServer, make changes to the translation externally and then import the TMX file back the TM, new TM entries will be added to the WorldServer TM. The previous TM entries will
not be replaced.
What you can do is to delete the existing entries before you import the new ones. However, be careful when deleting because deletion cannot be undone.
For instance, if you are planning to send your language provider only the Chinese language of a multilingual TM, make sure to run a Search for that language only as explained in this
article (section "To export all Entries for a specific language only:").
Once exported, you could use the button
Delete Result Set to delete that Chinese TM. Since you have previously exported it, you have a back up of that portion of the TM (the chinese portion). Finally, import the changed/reviewd TMX file from SDL Language Services back into your WorldServer TM. Now you will have only the entries that have been reviewed externally.
This procedure can be applied to any Search result: use
Export Result Set to export only the TM that you want to review, and use
Delete Result Set if you do not want the old TM entries to remain in the TM.