If you consider the influence of Mesopotamian creation epics and stories on Biblical authors, what are a couple of difficulties in adjusting the creation story from polytheism to monotheism?
All major world religions, whether polytheistic or monotheistic, have creation stories. The common characteristic of these creation stories is that, to phrase it technically, they narrativize cosmology. In other words, they explain the structure of the world by telling a story about its origins. This is equally true of Navajo, Hindu, Judaeo-Christian, Sumerian, and Greek creation stories. The main difference between monotheistic and polytheistic creation accounts is that in monotheistic accounts, usually a single god does all the work of creation, although that single god may create or organize various helpers, while in polytheistic accounts, creation may involve a single god who then creates other gods or may involve a team effort.
Revising a polytheistic story into a monotheistic one is not a particularly difficult or unusual task. Either the monotheistic redactor can simply eliminate all but one god, assigning all divine actions to the remaining single god, or the redactor can demote all but one god into demons or heroes or other forms of quasi-divine status.
Take the Sumerian account of creation:
In the first days ... earth had separated from heaven,...
... the Sky God, An, had carried off the heavens,
And the Air God, Enlil, had carried off the earth.
To make this a monotheistic account, one could change this to read "when the God An carried off the heavens, and his servant Enlil carried off the earth", demoting Enlil from a god to something like an angel. One could also have An carry off both heavens and earth.