saving changes is not permitted.the changes you have made require the following tables to be dropped and re-created

Mainly the cause behind this is ,SQL prevent saving changes that require table re-creation

If I am not wrong do you guys facing this kind of pop up while saving a table,which you have build already.
below pop up shows that you can not update the design of table, usually this function work in bulk Professions,where Project head need the developer to do test on his table without modifying anything from database.

ok after this,lets come to the point.
just follow the below procedure...
which means go on TOOLS then OPTIONS.
(Note:before proceeding please close all opened window under sql window)

Add caption

After which you'll get this kind of screen of OPTIONS(below image),now click on Designer on left hand side of window,then table and definition designers...
in which you just have to UNCHECKED a option which is

• prevent saving changes that require table re-creation.
  (like below image!)

that all! you done it ,and now can modify your table whenever you require.