Translation:

I'm using Firefox 3.6 and Zotero 2.0rc5 with WinWord 2003 (SPANISH version). I'm using "fields." I upgraded from 1.0.10 , and now, when I convert all my documents written before, I lose the bibliographic reference (all the previous citations appear as "item doesn't exist in the library". But in reality, the items are in My Library...but after the conversion to Zotero 2.0 it does seem to recognize the old documents, that were written with Zotero 1.0.

Actualicé primero Firefox de una vesión 3.5 (creo que 3.5.7) a 3.6; luego actualicé zotero 1.0.10, a zotero 2.0rc5. Allí me reconoció la biblioteca completa que ya tenía. Luego abrí un archivo ya escrito y ahí tuve el problema. Tengo back up de todo como para volver a usar zotero 1.0.10 si fuera necesario.... por un tiempo... ya que necesito seguir escribiendo y usando zotero para los archivos "en uso".
Alguna vez, hace mucho, había usado la opción "exportar biblioteca". Pero hace mucho. Generalmente cambio de computadora, y llevo manualmente la biblioteca reemplazando la carpeta C:\Documents and Settings\David\Datos de programa\Mozilla\Firefox\Profiles\biz0pvc3.default\zotero
Gracias por responder tan rápido!

hmm parece todo en orden - la manera de trasladar los datos entre computadoras está bien como lo describe droldan.

I think you're probably describing this issue, noted on the 2.0 page and the plugin installation page:

Documents used with the Zotero 1.0 plugins must be upgraded to work with Zotero 2.0 on the same computer on which they were previously used. If you upgrade them on a different, synced computer, you may end up with mismatched citations.

Using sync is certainly fine, but you just need to upgrade and re-save 1.0 documents on the same computer on which they were last used before the Zotero 1.0->2.0 upgrade. You can then copy the upgraded documents to the other synced computers.