It doesn't seem normal at the stage this case is at but, then again, we all know that there is nothing normal or routine about this entire matter. My take is that Flynn is now represented by a known White Hat or publicly Partisan counsel whose appearance may have raised unnecessary eyebrows if retained originally. Or perhaps his original counsel had no idea about the real dynamics going on and knew they were in over his/her head and begged out, i.e., they knew Flynn had not disclosed material facts to them. The most 'plebian' excuse for such a move would be if prior counsel was not being timely paid. This, of course, could be true as an "impoverished" Flynn was necessary to pull off the operation.
Seems that for opsec purposes having honest but "in the dark" counsel would have been the most prudent course. The only other reason I could think of is that previous counsel either realized how dangerous the game was, or that he/she (or family) was "touched" in some manner by the Dark Force.