How troublesome is it for a professor to replace a document posted on D2L?
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.