When you change a Page property in the Visual Studio's Elements Explorer, the changes are not applied if you don't have at least one test opened which contains an element from that Page. Visually the Page property is changed, but if you close and reopen the VS solution you'll notice that the changes you made are reverted back to their previous state. However if you have a currently opened test which contains an element from that Page, the test will be marked as 'dirty' and the changes will be saved succesfully.