The iPhone X and iPhone XS have the same dimensions--5.65 x 2.79 x 0.3 inches--but there are some important differences. Notably the camera lens on the iPhone XS is larger since it now features a wide-angle sensor. The increase in size is significant enough to make your old case not fit quite right. The camera bump protrudes a little further out as well, but not by much: about a tenth of millimeter in extra height.
There are also other things to consider, such as the speaker holes on the bottom of the phone. The iPhone X has a uniform number of speaker holes on either side of the charge port, while the iPhone XS has only three to the right and six to the left. So the case will certainly fit, but the cutouts won’t be aligned.
Those changes are enough to make it a prudent decision to upgrade your case for the iPhone XS. Luckily, totallee has their signature thin cases available for the iPhone XS which have been specifically designed to accommodate the larger camera, and have precise cutouts for the bottom speakers. The iPhone XS cases comes in four matte colors: frosted clear, frosted black, solid black, and navy; one transparent color: clear (soft); and a black leather style. You can pick up an ultra slim case for $35 or the black leather backed case for $45. Still not convinced? Check out this article on why you should buy from totallee