That discussion has occupied the time of many an airline flight deck. Should you get a TTF inflight if the fuel you departed with was legal? There is no requirement to update a TTF if the one you based your decision on was valid for your arrival. By updating inflight and if there is now a requirement you have just sold yourself a hospital pass. If you don't have the fuel then you must divert or calculate a PNR and hope for an improvement in the weather. Don't tell don't ask is the underlining principal.