Changes on site

How much will it be necessary to change an algorithm, if a site with which a robot works, visually changes location of blocks? And if I update any content on the site?, it depends on the number of changes on the site and the specific situation. The robot uses HTML-code to work with the site, or rather selectors of elements of the site (in the form of XPath or CSS). When you change the size of the site or its blocks may change the layout, and hence the selectors, too.
In general, you can choose selectors so that changes to the site minimized the impact on robots.