You can transfer the data from your old mob. to the N73 including your contacts , inbox , gallery , ...... , and also remove the installed themes from N73 memory card by the following procedures:
1- For theme removal from memory card : when you connect the USB cable to the N73 it asks you about the connection type (normally we select "PC suite"😉, Select "Mass Transfer" where your PC will recognize your N73 as a card reader, so you can see your memory card and all the hidden files and folders.
There you can find the installed themes in "Private" folder and you can delete theme manually.
2- For contact and data transfer there is an application installed on the phone by Nokia (default from the manufacturer) named "Transfer" in the applications folder, run this program and it will contact any other nokia phone (through Blue tooth or Infrared) and pull all the data from the other phone after asking for permission.
these procedures i have tested by myself and they are perfect.