Make sure you export all contacts. Some phones I've met would export different groups of contacts separately, e.g. phone-only contacts, Google contacts, etc. That's fine as long as the import replaces contacts, but if it doesn't and you heed to clear things to sort it out you want to be sure you have a vcard backup of absolutely everything before doing that (if it exports all contacts into a singe vcard file then your original file is your complete backup).