1. Login to your phpMyAdmin, export the Joomla 1.5 _users table as CSV for MS Excel with tick on Put fields names in the first row

  2. Open the exported file with MS Excel.
  3. Delete the gid column and save the file as CSV.
  4. Go back to phpMyAdmin, select the Joomla 2.5 _users table.
  5. Click Import, select Format as CSV, change Fields terminated by with comma (,) instead of semi-colon (;) to avoid the error while importing "Invalid field count in csv input on line 1"
  6. Click the Browse link and check the imported data.