So let's say you have the Noterth Campaign Setting and it has an overview of Merrydale and the Probably-Not-Haunted Forest. You enter that general information into your realm, no problem. Later you pick up the Merrydale supplement and it goes into a lot more detail. What do you do?
Do you keep the old snippets? Do you replace the old snippets, where the new information is more detailed? Do you keep both but note what books they came from? Am I thinking too hard about something that shouldn't be this hard?
Edit to add: What if you have both books from the start? Do you even bother with the Campaign Setting blurb, or just hold off until you have time to put in the supplement?