It sounds like you're mixing together several features in Marketo, which makes your question hard to parse.
Adding new Segmentations doesn't affect any existing part of Marketo. Adding Segments to a Segmentation -- if the Segmentation is in use -- means you have new dynamic content to fork from the Default, but then again that's why you'd add a Segment in the first place.
In your footer example, if you use a Dynamic Snippet (itself tied to a Segmentation) you can make changes to the Snippet alone and they will take effect everywhere.
I would imagine however, that if you delete a segment from a Segmentation it will wipe away existing dynamic content for it. Although I am doubtful Marketo would allow you to delete it if in use.
Although I am doubtful Marketo would allow you to delete it if in use.
You can delete a Segment even if the Segmentation is in use, and regardless of whether that Segment's dynamic content currently differs from the Default.