Short answer: yes, it’s a redesign, the functionality is better, range is pretty much the same, though it is smaller.
Longer answer: the flexNExT had a propensity to fail quite often because of the antenna design, the flexMN is a response to that issue. It uses DT’s more usual copper antenna design as opposed to the bulleye’s foil antenna, which makes it more robust and (hopefully) not as easy to brick, and because of this, the flexNExT isn’t available for purchase anymore, as ItaBeAight mentioned. You should know beforehand that you’ll need a proxmark3 to set up the chip and changes its emulation, and that some people (myself included) have had problems getting phones to write to it. It’s not a problem using the proxmark3 to write to it though, and once the chip is set up the way you want it, it works exactly as it would as another chip (reads consistently with good distance on an acr or from your phone).