In both cases, the updated corpus will be used for future updates.
When removing a source, all of the articles in it's history will be removed in tracked topics and analytics apps within a dashboard's retention period.
When adding a source, the functionality depends on the source. Some sources include a history of up to 100 articles. Other sources only include 1 article in it's history.
You've been using Netvibes for a month and have always been using 3 sources (A, B and C) publishing 1 article a day. So for the past 30 days, daily mentions are at 3. Today, you add a 4th source (D) which updates at the same pace: one article a day. Here's what happens: Netvibes will continue to show you 3 mentions for the past 30 days and will start showing 4 mentions when it updates within 20 min.
Similar to the previous example, but now instead of adding D, you remove B. Here is what happens: Netvibes will remove all the articles from source B within the dashboard's retention and update the analytics chart accordingly.