I had to help a friend in converting his 700 contacts from a nokia phone to his samsung phone.
It can be done easily if you have a computer and some patience.
First download PC suites for both the mobile phones which can be found on the manufactures website and install them.
First connect the new phone to which you want to transfer the contacts to. See what formats does it allow the contact to be imported into.
Then connect your old phone and using the pc suite of that phone provider export the contacts.
Which usual will be in .csv format. Make a copy of that file and back it up some where safe.
Then import them into the new phone PC suite. This is where you get in to trouble you will have to do the import few times until you get the contact exactly the way you want.Because both manufactures use different phone operating system and have different contact fields. It will take few trial and errors before you can get all the fields right. Once you get what you want sync it back to the new phone.If you have the latest phone with gprs you can also transfer contacts by syncing them to an online provider like, Yahoo or Oggsync etc.If you are doing a contact sync between two nokia phones you can also use a free service from nokia called Ovi.