All you really need to do is unplug the old drive. When you plug it back in (after installing on the SSD) Windows will recognize the new drive as D, E, or whatever letter is free after naming all your optical drives. No need to worry about letter management. Even if the old drive has been explicitly set as C (which isn't the default), the OS will just ignore the setting if there's a conflict.
Desktop: i7-4790K @4.8 GHz | 32 GB | XFX Radeon 6950 | Windows 10 x64
Laptop: i7 740QM | 12 GB | Mobility Radeon 5850 | Windows 10 x64