What happens to the content when I modify or delete a smart tab?

Modifying a smart tab will adjust the selection of content that will be displayed in this tab according to the (new) filters you will appply.
Deleting the smart tab will simply remove this view from the space menu.

In both cases, you will not lose the associated content.

Deleting a space, however, will delete all the associate content.
Be careful with this option.