A family of Microsoft word processing software products for creating web, email, and print documents.
As Stefan says, you can't switch a table caption to a figure caption or vice versa in the Captions dialog. But you can do it simply by editing the existing caption in the text. The only differences between the two types once they've been inserted are the words "Table" and "Figure" at the start, and the identifiers in the SEQ fields that display the numbers.
So rather than going to the effort of deleting and recreating the captions, just press Alt+F9 to display the field codes, and edit them and the words at the start of the captions. Then press Alt+F9 to return to the field results, and press Ctrl+A followed by F9 to update all the fields.