I believe you are only changing the URL. In that case, when we add a service or web reference, there is an entry made in app.config or web.config file which points to service URL. You can simply change that. An example app.config file the a web reference will have an entry like this:
<applicationSettings>
<ConsoleApplication1.Properties.Settings>
<setting name="[SettingNameHere]" serializeAs="String">
<value>[ASMX URL here]</value>
</setting>
</ConsoleApplication1.Properties.Settings>
</applicationSettings>