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Apply for a visa from Spain, documenting your entire trip, and explain that you will not be making the trip to France your existing visa was issued for.
The Spanish consulate will most likely revoke the French visa.
However, if they instead decide to issue a new visa with back-to-back validity, that will work just as well for you.
Either option will be valid for your plans, so do not worry about trying to tell the consulate which of them to use.