This happens when you're in the SSMS gui making changes to the table that either affect the behavior of an index, cause existing data to be truncated/nulled, etc. This can also happen you try to change the order of columns, or change datatypes.
If your credentials allow it, you can get past this message in order to truly shoot yourself in the foot! :)
- Navigate to Tools --> Options
- Expand Desingers
- Click on Table and Database Designers
- Check the box Prevent Saving changes that require table to be re-created.