When you change the file extension of a file, be it on Windows or Linux, the only thing that happens is that the file's filename will be changed.
The file extension is part of the file's filename, so when you change the extension, all that you're really doing is renaming the file. The actual data inside the file doesn't change, just as it doesn't change when you rename it without changing the extension. This means if you change the file extension and change it back right away, you won't have changed anything.
Changing the file extension can't damage the file, HOWEVER, there are some cases that you can end up damaging the file after changing its extension. See ["If you change a file name extension, the file might become unusable"] for details.