As noted by others, it’s always better to discuss with the airline. Choosing to unilaterally curtail the trip and assuming you “would” have been sufficiently late for compensation makes you far more vulnerable. Once you discuss wit them and they highlight the options you can consider the alternatives including journey in vain, but if you choose to voluntarily transform your fra-cfe into fra-cdg, assuming that the delay to cdg is under the compensation threshold, you can’t expect them to pay easily based on hypotheticals. Things could be slightly different if your flight is cancelled rather than delayed.