The auto-replace option can or even should be enabled when:
if at least one proxy on your list has "Not Working" status (in this case, put a checkmark next to the "Proxy is not available for more than 15 minutes" option);
if the website shows the country wrong, the replace option "Proxy has Low Country Confidence" will replace these proxies with all high-country confidence proxies automatically, as the proxies on the account are a mix of low-country and high-country confidence proxies;
your proxies are working slowly, put a checkmark next to the "Proxy may be experiencing temporary slowdown" option.
If you have proxies selected on your plan from a specific country, they will be replaced with proxies from the same country (e.g. if 10 UK proxies are experiencing a temporary slowdown, they will be replaced by other 10 UK proxies, not proxies from random countries).
In these cases, the proxies will be replaced free of charge. These options can be enabled or disabled whenever you wish to do so.
If you don't want this option to be enabled at all times and have static proxies, please disable it and Save Changes.