Supposedly, that "change something more than 10% you can call it your own" is an Internet myth- if the other 90% of the model is made by someone else, than you CAN NOT claim it as yours. If you scanned in a copy of the Mona Lisa and recolored it blue and changed the background, could you claim that you made the artwork from scratch? Um, no. Changing something by 10% means you altered something- it is not an original creation by you nor can you copyright it as belonging to you. In fact, if you change a mesh by adding polygons or subdiving, you may change it "more than 10%", however, people in this industry are very sharp- they will see that your model is a derivative of another model and will jump all over you for mesh theft. So, whether you are "allowed" to do it becomes irrelevant- your models will be removed from sites and you will branded a thief.