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It is no secret that tire rolling resistance decreases as tires age. Mostly this is due to tread wear and overall thinning of the crown. About 2/3 of tire rolling resistance comes from compression and bending of the crown. Rubber is an inefficient spring in that it does not restore all of the energy used in straining it. Some strain energy is diffused as heat and the difference between applied and restored energy is hysteresis loss or rolling resistance. A thinner tread compresses less so it suffers less hysteresis loss. You could shave or "buff" new tires, reducing tread thickness to decrease rolling resistance but it is difficult to rationalize any ROI due to the decreased service life of the tire.

Another smaller factor decreasing a tire's rolling resistance as it ages is the natural age-stiffening of the rubber compound. A higher modulus compound reduces total tread compression and therefore strain hysteresis loss (rolling resistance).

You might pre-buy your tires a year or two in advance and age them yourself in some very hot outdoor facility as a steel shed, sheltered from UV and test strategy for yourself. It is difficult to say definitively how much this compounding hack will decrease rolling resistance as compounds vary and some may be more resistant to aging factors than others.