You have a few options to achieve this.
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Just enter the new flight plans in manually, call the fuel truck (or just use the weight and balance option on the toolbar) to adjust fuel / cargo / passengers / etc…
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If the aircraft you fly supports it, import your flight plan from the supported source
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Ditch the in-sim ATC and use something like Pilot2ATC, VATSIM, etc… that does not require the world map.