How troublesome is it for a professor to replace a document posted on D2L? On many occasions I've had professors that will post assignment instruction documents on "Desire 2 Learn" or D2L, realize they made a mistake on those instructions, and tell us students either in class or via school email that they had made a mistake and what to do about it. 

I have never once seen them simply replace the document itself.

I am not here to complain. I have heard that D2L is difficult to handle, and I've seen many professors get confused by it. I am just curious if D2L makes it difficult to replace files that have been posted, or if there might be some other reason why they don't bother updating their documents.
It is not that hard to change a file in D2L. 
Click a small sign (looks like v) right side of the file name, choose “ change file”.   That’s it.

I guess your professor worries what if there is a student(s) who already printed the document out and never go back to D2L since he/she has a printout. 

Indeed, D2L has an option “notify students” when the instructor change a file in D2L. Just one click is required to sent notifications to students that the file is updated.