I have seen and faces following issue most of the time .. when you are going to alter the table ..
Saving Changes in not permitted. The changes you have made require the following tables to be dropped and re-created. You have either made changes to a table that can’t be re-created or enabled the option Prevent saving changes that require the table to be re-created
If such situation arises, then this feature can be turned off by going to Menu >> Tools >> Options >> Designers >> Uncheck “Prevent Saving changes that require table re-creation.
This restriction only applies to operation attempted using only SSMS. If modification of tables are attempted using T-SQL in Query Editor, it will work just fine!