The Law of the Land?

All the way up to the October 1 launch of Obamacare (Affordable Care Act), whenever the Republicans tried to introduce or pass a bill that would delay or defund this law, we were told by members of the media and several democrats that it was the "Law of the Land" and nothing could be changed without approval from both houses of congress and the signature of the president.

My thoughts on that excuse were that at one time, slavery was the Law of the Land, lack of voting rights for women was the Law of the Land, segregation was the Law of the Land and they were changed.

Now, suddenly with the prospect of incumbent democrats losing their seats in the 2014 elections, the president feels that he can make more changes in the Law of the Land without congressional approval.

Can someone out there in cyberspace, or at the NSA, please explain to me why Obamacare was the Law of the Land in the summer before it took effect and now why this fall and after it has taken effect is it perfectly permissible to change the Law of the Land.  Or is it just because the president's approval rating is sinking faster than the Titanic and he has decided to change the law, yet again?

'Tis a puzzlement.