So, living in the Cedar Valley corridor of Iowa and seeing those 100 year floods happening quite regularly now, as well as the 1000 year floods, how do you start calculating the chances of a 10000 year flood being more likely relative to the changes in the 100 and 1000 year floods,

Not sure I understand this question, but I will say that it seems more and more likely to me that the probability distributions used to extrapolate extreme rain events from available data aren’t describing the tails properly. The biggest example of this was Hurricane Harvey. I’m not at all convinced that Harvey was the worst rainfall to fall on Texas since the last ice age.

and how do you calculate the increasing peak levels based on increasing volume of precipitation? I just bring it up because I don’t think municipalities want to go there….

One thing that is happening, is the development of watersheds changes their runoff characteristics, particularly the addition of impervious areas from urbanization. This is one of the primary reasons flood maps must be regularly updated.