What is difference between Nexium 40 mg and Esomeprazole Mag DR 40 mg? My ins company switched me?

I also have been using Nexium 40mg twice a day for years, also my spouse. Near the end of last year my insurance refused to cover me for twice a day and my doctor made it once per day, I get Nexium now once per day. Then two months ago, my spouses insurance issued letter saying generic was avail and we would have to change, which we did. Now this month we get a letter saying spouse no longer covered for twice a day, only once per day. Our doctor at first was set on non generic, but gave in, and kept it at twice per day. We called MD office, we have appoint but MD said it should remain at twice per day. We called Ins, they say no way, only once per day. Who has the control here? The MD or the Ins? I am gonna go out on a limb here, and say that the Ins will prob win out here ... what is wrong with our Ins system? How can they override the doctors? This is crazy!