Suggest you first COPY itunes from drive C: to Drive D:
Then test out thoroughly using the files on Drive D:.
Then when you're totally confident that all is working OK, DELETE the relevant files from Drive C: - maybe playing extra-safe and doing it in chunks rather than all in one go.
Then (finally!), empty your Recycle Bin.
This is called the 'belt, braces and bits of string' approach. :-) HTH.