Not sure I fully understand the question but the BGP router-ID and BGP peering address (including Loopback address) are two different things. You can basically set the BGP router-ID to any IP address - it does not have to be routable on your network, it is merely used to identify the router. The reason the router-ID has the same format as an IP address is that if the router-ID is not explicitly set, it can be derived from the highest Loopback or interface IP address. The BGP peering, however, is configured using real reachable IP addresses. There are different ways of configuring the BGP session. You mentioned setting up an eBGP peering using Loopback interfaces. Configuring a new BGP session using the update-source command would not affect the router-ID.