I have frozen corned beef before cooking many times, and it will be just fine after thawing and cooking as directed. I just leave it in the package and stick it in the freezer. That's it. I am not a fan of reheated cooked meat of any kind that has been frozen and thawed, it still tastes dry to me.
You can freeze cooked beef without a problem. I frequently do it. I often grill beef, thinly slice it and put enough slices in plastic sandwich bags for 2 sandwiches before freezing it. Several such bags are put into a larger resealable bag before freezing. When I wish to have a beef sandwich I defrost a single bag.