need old 2 pipe \"upfeed\" system to hot water help

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Per the gravity hot water heat section in the Heating Q&A portion of this website, it shows the upfeed system being a direct return 2 pipe system. Direct return meaning that the first radiator to get heat is the first to return to the boiler. This as opposed to a reverse return where the radiator to get heat first is the last to return to the boiler. If a new boiler gets installed , how do I repipe the near boiler piping to prevent this system from being unbalanced due to the origanal direct return piping ?