I would doubt that EOTech has a conversion pack to upgrade an "N" cell sight to a "AA". But, a call to EOTech would answer that question.
I personally wouldn't buy an "N" cell sight. I have two EOTech sights now, both "AA" models. However, if the price is right for the "N" cell model you are looking at, go for it and buy a stash of "N" cells to keep it running.
Martyn