Rev Proc 2017-57, sec. 3.02:
So, I think this would have to be considered a change in funding method (since it's not a change in data or actuarial assumptions), and I also think it would probably qualify for automatic approval, assuming you meet all the requirements of 2017-56 4.02. The only way to know for sure though would be to apply for approval for a change in funding method.
Edit: I went back and re-read your original question. I had been thinking that you were asking about switching from assuming mid-year to using exact dates, but I realize now you were actually asking about going the other direction. That could present an issue for automatic approval, since 2017-56 4.02(5) requires that the new software be "designed to produce results that are no less accurate than the results produced prior to the modifications or change." Assuming all cash flows occur mid-year would seem to be, by design, less accurate than using exact dates.