Depends on what mod, If you play Silent there won't be a problem!
If you play Jaymod or any other mod it will be a problem, So best thing is to make a copy of your ETkey which is located in your ETmain folder.
You can easily get into that folder by, clicking on your Wolfenstein EXE file. By pressing the right mouse button and click on Map Location or just go to C:\Program Files (x86)\Wolfenstein - Enemy Territory . <- Depends on that path you installed ET. Once you are in the ETmain map, make a backup of your ETkey. By doing this you can once you have your new computer easily install ET and put the file back in the ETmain folder.
Good luck!
EDIT: I have uploaded a image how the file look like. (Attached files)