"Save changes is not permitted" when changing an existing column to be nullable

I've found the solution. Go to "Tools > Options > Designers > Table and Database Designers":

I had the same problem; wanting to Allow Nulls for a column that previously did not. Consider MS's warning to NOT turn off this option: http://support.microsoft.com/kb/956176

And their recommendation to use Transact-SQL to solve the problem, e.g. alter table MyTable alter column MyDate7 datetime NULL

This solved it for me.

It's a setting in SSMS.

Tools - Option - Designers - Prevent saving changes that require table re-creation