I created an arc using the circle tool and then changed it to an object. Can it be changed back to an arc?

Assuming you mean that you converted it to a path, then no, it can't be converted back. Conversion to paths is a strictly one-way affair. The best you can do is re-draw another arc, using the existing path as a guide. If you copy the existing path to the clipboard you can then paste its style attributes to the new arc using Edit > Paste Style, if necessary.

The general rule is that you should make a copy of any object that you wish to convert to a path, and move it off the side of the canvas, or hide it in some way. That way you still have an original to play with if you need to make changes in future. This is especially important when converting text to paths.

Thanks. Good idea to save the shape.