The easiest way to switch from an IP to a URL is to rerun the aggregate installer so that the configuration files get properly configured to use the domain URL instead of the IP address.
If you want to manually edit the files are located in ODKAggregate-settings.jar. However if you mess up those files aggregate will not boot.
The answer is actually the same as your previous question as aggregate takes the IP address or the URL you give it in the config file (normally input by answering a question in the aggregate installer) and treats that as the preferred URL when creating it's response string. So you can switch IP address or you can switch between a URL and IP address using a similar method.