I don't think there is a restriction of how many times you can switch after 180 days and the I-485 approval date. What is important is that you have a job at the time of approval and that this job is 'indefinetly' ('indefinitely' in a way that it is a fulltime job without expiration date). Definitely, use the search function of this board to see other opinions, I am sure that this question came up before. You probably have a lawyer already so that you a prepared in case of RFE. Ask him/her. If you feel that he/she has not enough experience with AC21 then contact Sheila Murthy or Rajiv S. Khanna; they have handled several of these cases successfully. This is simply too important to do without a very good lawyer.