You can upload content created by others only if you have claimed it in the DLS Cleanup Process.
You need to change the Kuid number to the next version and this is not always easy. As mentioned using Notepad is one way. What I do as I have a number of versions of Trainz is to repair items in 2010 and test them there. Then I change the kuid number, so if it is a "kuid2" version I increase the versions number by one, "kuid2:123456:1234:2". Now the item is open for edit, I go to TS12 and import the content from 2010 into there. I then test it in TS12.
Once I am happy and the item has been committed I can highlight the item, right click and make a CDP file, saving it somewhere I can remember. This CDP file can now be uploaded from the DLS Cleanup Page.
For the old item in 2010 that is open for edit, change the kuid number back to the original and commit it.