I believe that 99% of the cause for Dixie Landings becaming Port Orleans French Quarter has to do with PCness. This evidenced by other things previously mentioned and the fact that select additional components of DL were renamed:
*Colonel's Cotton Mill -> The Riverside Mill
*Cotton Co-op Lounge -> River Roost Lounge
*Dixie Drive -> Riverside Drive
*Dixie Landings -> PORS
*Dixie Levee -> (new name which I am unsure of)
Additionally, a painting of a cotton blossom on "The Riverside Mill" sign inside has been replaced with a less-recognizable flower.
Pretty much all references to Dixie or cotton has been removed, and with good intents. If it were for any other reason, why would only these names have changed, and not more in order to more seemlessly connect the two resorts?
Of course, the word "Dixie" has its origins in the French. "Dix" or "ten" appears in large type on 19th Century notes issued in Louisiana. Recall that French was the prominent language in some states prior to The Louisiana Purchase. The South later adapted "Dixie" as a nickname for the Confederacy.
The other 1% of the cause could have to do with Dixie Landings having been a more popular resort, the more popular Doubloon Lagoon pool at PO(FQ), or simply to combine two resorts which were already sisters

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Sources:
Walt Dated World. 05.21.2004
http://www.waltdatedworld.bravepages.com/id147.htm
http://www.syngraphics.net/pmdictionary/DICT-D.html