Reneged Offer Advice

Helo all.  I am looking for some advice.

I have gone through the interview process with a few companies and was recently offered a position from my top two choices.  Both companies are offering very similar compensation packages and the job description is somewhat the same as well.

I accepted the offer from Company A and was called 2 days later and told that they need to hold off on the offer due to an unforseen problem and they may or may not be able to offer me the position again after the first of the year.

I had not said anything to Company B until I confirmed with Company A that they had accepted my "Acceptance" of the position. (Very happy for that in hindsight).  After much consideration I am going to change and accept the position with Company B.

The advice I am looking for is:

A.) Am I justified in switching my decision since Company A reneged on the offer with no certainty that they would be able to offer me a position again.

B.) Best way to convey that message. 

I believe I am justified, and I feel bad about that because they were my top choice.  I need to leave my current employer and I feel that if i try to wait to see if A is able to offer I will lose the opportunity with Company B and ultimately left with nothing.