Good day.
Please clarify your case:
1. Do you publish a stream to WCS from RTMP encoder
rtmp://wcs:1935/live/stream
?
2. Or do you republish a stream from WCS server to a third party RTMP server?
In both cases, you cannot publish a new stream with the same name until previous stream is stopped.
But, if you want to replace stream content temporary (advertising, for example), you can publish stream with another name, then inject it to original stream. In this case, you don't need to stop the original stream.
Please read the details and known limits
here.