I agree it was lazy/sloppy (former IT guy here). I also agree you should fix it now rather than have a bigger mess later.
If you rt-click on your start menu and select Disk Management you get to an interface that should allow you to fix it, although you can also do some damage here so proceed with caution.
The disks will be numbered, starting with 0 and then for each disk you can see any partitions and what drive letters are assigned to what partitions.
It sounds like you need to find your old C: drive, rt-click and select Change Drive Letters and Paths...
From there you can remove the C: assignment and then add another available letter. G will not be available yet because it is already assigned to your new SSD.
Now that C: is freed up, you can assign that to your new drive via the same method and remove G. Then if you want you could go back to the old drive and change it's letter to G if that is what you want.
Don;t do this without a current backup, just in case. There might be undesirable side effects and you might need to revert. If there are a lot of things looking for their files on G: that are now on C: instead, you may need to reinstall some applications and possibly Windows to get it all working right.