If you care at all about genealogical accuracy, do not download a gedcom from WorldConnect or whatever site to merge with your tree.
Know that practically no vital
English records exist before 1583, and material purporting to be back to 1400s or earlier, from whatever country, are very suspect.
The advice to not add it unless the site gives 'source' was rather hastily stated. Anyone can circulate falsehoods in trees, gedcom files, emails, web sites of many types. Many books have been published with **totally fictional** genealogies that have been endlessly copied into trees on the web.
No genealogical program, incuding FTM 2009 will allow just partial over-writing of your tree. It is total replacement or add a newly-named tree. For more on what FTM will do, including details on differences between various versions, do internet search for reviews & discussion, especially by some well-known genealogical bloggers, and check the discussions under topics/genealogical software on these message boards.