updating new user attribute based on old user attribute

     Someone converted multiple accounts and forgot to grab the telephonenumber attrib during the migration.  Now I have 300+ users without any telephone attrib at all.  I am wondering what's the easiest way to populate the new user telephonenumber attrib based on the old.  

     After some manual labor in excel, I can get the new user account and the old phone number into the same csv.  but thats where I get stumped.  I'm not sure how to update a user account with the corresponding property in a csv file.  And there may be an easier way to do it all PS, ie grab the old account telephone attrib and pipe it into a set-aduser.  So if updating it via the csv is not the right way, let me know.  TIA for any replies.

NewUserAccount        OldPhoneNumber

a12345                           972-531-5432

a23456                           214-323-4567