Just to agree with Nate1976 (again).
If it is released here, it is placed in the public domain. You may do what you will with any of it. Generally it is considered good manners to give credit to the artists who's work you're making use of. This is not strictly necessary, but almost everyone has always done it this way. The only thing that is not allowed is to take credit for others work.
For example: If you wanted a hovercar, and couldn't find one... and you decided to use my A-Wing as one. That is fine. If you say you made the hover car, not ok. If on the other hand you modify my a-wing to look like a hover car you may say you made it, but it is nice to give credit to the person who did the base version.
So see how I handled this, follow my link and look at my ME-262 and Jeep. Both are modified versions of other peoples work. In the case of the jeep, it was almost entirely re-done (720 of 776 frames were completely new, and the remaining 56 frames were heavily edited... so 96.4% new), but I still gave credit to the guy who's work I was modifying.
Similarly I changed a building by Ukas, and made sure to give him credit for the original.
--==short version==--
You can do anything you want with any thing here, just don't forget who made it.