and reinstall on D. then do a disk clean up on Drive C.

If they are just partitions on the same drive (probably not but possible) you can use a partition program to change the size of the partitions (expanding C and shrinking D)

If it turns out they are different drives, just uninstalling reinstalling on drive D: is probably the easiest method for most programs
but there are also some utility programs you can get that will move some installed programs from the C to D drive for you.

It may be possible for the operating system to move the program for you, for example on Windows 10 see:
https://recoverit.wondershare.com/harddrive-errors/move-programs-from-c-drive-to-d-drive-windows-10.html

You'll need to become familiar with Windows Administrative Tools in the start menu.

First go to Computer Management then on the left, Disk Management and give it a moment to populate the list. You'll find out how many physical disks you have and what partitions they contain.

It is possible, using inbuilt Windows tools to shrink or extend partitions on the same disk. Post a screenshot of your Disk Management window if unsure.

or you might end up on the wrong side of zero

I wouldn't be bothering about trying to move programs around. This will play hell with your registry and believe me, it's not worth it. Just un / reinstall in the right place when you have the partitions worked out.

Other than maybe booting a second or two slower, it's running well and I don't want to wreck anything. Appreciate the advice!