I don't think replacing Ash with May would have been ideal, but it would have been the most ideal time out of any other. He had a fairly fleshed out arc, something of a protege to fill his shoes, and hadn't solidified himself TOO much into the series just yet that people couldn't fathom him being replaced (after Misty and Brock were proved dispensable, it might even have been expected at that point). It wasn't necessarily the best idea, but if they had wanted to take that direction, then I feel that would have been the best time. Now I just don't see it transitioning well. Another Pokemon project with a different protagonist sure, they proved that, but the main anime, I think Ash is too established into it.