you can flip the letters around easily in mounted devices in the registry.
they of course wont change while the system is UP, because any pointers to it would fail.
BUT
if you do not have a whole boot ability and operation via D, and ANY aspect of booting or the system is on C , then just switching the letters alone would do you no good.
ntldr and boot.ini on the root of both C
These items ^ on the disk are where the "boot" originates, this is the first thing that the bios passes your computer off to. from THERE the boot.ini points to the actual whole system.
the system allows for this so this can happen, and it can operate like that fine forever, it is just plain stupid (of the system) though when for most users this is the last thing they would want.
but that is how they get it to boot to things like the second partition.
Further explainations I cannot do well enough , because you can loose boot if you dont do it correct.
AND
If any programs were using the "D" designation in thier paths these programs and connections would resolve to the wrong places. bloody mess depending on many things.
We would switch this back and forth after a clone to another disk/partition , BUT we would only do this:
a) when the install and everything was originally on the lettered C: item, and the partition was hopped to a new location.
b) before any additions/changes of programs were made to the system.
c) ONLY because the disk was cloned from place to place NOT because it WAS D: , because it was C: before the clone.
When you change the drive letters:
there does exist programs that will attempt re-pointing after a letter change, they will search for the letter in the registry, shortcuts, and text type config files and change it there ALSO, attempting to keep everything synced to the other letter change. (they only worked about 90%)
We never applied this , because we always tried to Keep the letter as it once was.
Gosh i hope that helps because all of it is more complex than I am :-)
as stated in the first comments, check your boot.ini both disks for your boot parts and pieces. check your disk manager for your actual locations. and check any pointers that are already pointing to D. you dont have a big problem (it will work fine like that) , and changing it fully and completly is going to take more Danger than switching a letter.
mabey my editor can come along and fix that post :-)
