The answer to your first question is, unfortunately, yes, CPS can change their goal at will. I would point you to CPS Handbook provision 6211.1 that explains the statutory requirement of concurrent planning. In short, this means that you were one of at least two plans CPS had for the children and they were working on both plans at the same time.
The answer to your second question is also yes. You should definitely seek the advice of a competent family law attorney in your area that regularly works with CPS. You may have rights and, therefore, legal options available to you.