When we have a doubt regarding a biblical command, we take a stringent approach. This is known as ספק דאורייתא לחומרה.
When we are obligated to do something at a set time, and especially when that time is dependent on another phenomenon or event, it is imperative that we know when the appropriate time is. This is why two days of Yom Tov were established outside of the range to which the court in Jerusalem could send messengers notifying the public of the new moon each month.
In terms of these particular commandments, Millah and Pidyon HaBen, there is an added dimension, which is the tension between the urgency of the Mitzvah, which would incline one towards early action, and the potential problem of acting too early and thus not fulfilling it at all, since the set time hasn't arrived yet. Thus there is a question of how to be stringent - do you lean "early" so you don't miss a day to perform the Mitzvah (Millah carries Kareth if left unfulfilled; Pidyon HaBen does not, but it is mandated at a set time, and by pushing it off a day you miss that time), or do you lean "late" out of a fear that doing it too early is not valid at all? Which option is considered לחומרה?
In short, we do not allow premature Berith Millah, as it may be invalid. (I believe there is some debate about this, but this is how we hold.) The same concern exists for Pidyon HaBen. What is not a shared concern between Millah and Pidyon HaBen is the severity of delaying the Mitzvah.
Apparently, in some limited circumstances, unlike Millah, Pidyon could, in theory, be done on one day without a Berachah (possibly 'Al Tenai), and then again on a different day (possibly with a Berachah if the first had been 'Al Tenai). Physically, it would be impossible for Berith Millah to be done twice, obviously. However, this is mentioned as a possible solution where there is no proper Kohen available, not where timing is an issue.
Furthermore, there is a debate among Posekim as to how we calculate the timing, and the practice of performing the Mitzvah on the 31st day is a way to ensure that it is not done prematurely according to any opinion. In essence, we are already in a situation of ספק דאורייתא, due to this dispute, and we hold that חומרה, in this case, means delaying it by a day to be sure.
I have not seen a direct answer in a Teshuvah about this, but all the Halachic calculus leads me to the conclusion that we would delay the Pidyon HaBen until the Safek Day 31/32.