Just purchase & installed SimCity4 game today (1/17/2021) and now I am wanting to just change the My Documents files to the D drive. Is there a way without deleting the game and reinstalling it. I have much less space available in the C. The game is running but I am running short of space.

Try making a symbolic link.
It's the mklink command used from a command prompt.
Basically looks like this:
mklink /d source destination

google it

I use /J for a folder Junction, instead of a soft link. Either is acceptable, the difference is that a /D can be a folder or a file, /J can only be a folder.

so
mklink /J "E:\Games\The Witcher 3 Wild Hunt" "C:\Games\The Witcher 3 Wild Hunt"

Where the first location is where the link will appear, and the second is where the game physically resides (I put it on my C: because of SSD Vs HDD, most of my other games are on E:)

I my example, I'd move the folder from the E: to the C: first

You'll move it from the C: to the D: first. and then open CMD or powershell in admin mode and run the command line to create the link

mklink /J "C:\Documents\SimCity4" "D:\Games\SimCity4"

(note, the first is where the game WAS before you moved it, and the name of the folder needs to be EXACTLY what the game folder was before you moved it. The second one is the new location. You can also substitute /D for /J if you prefer to create a soft link vs a Junction.)

EDIT: Alternately, you can create a batch file and run it as an Admin. Mine reads as follows (NOTE: Lines with a * are commented out and not parsed by the system):

@echo off
* /J is for folder, /H is for a file, /D is for a soft link for folder OR file
* The original location is first (where the link will be); the new folder is second (where the actual files will reside)
mklink /J "E:\Games\The Witcher 3 Wild Hunt" "C:\Games\The Witcher 3 Wild Hunt"
exit

One more thing to note, if your folder has spaces you need to use quotation marks like the poster above did.