The rebuilding process utilized a used EMD 12-567A normally aspired prime mover from a retired E-unit, replacing the unreliable Alco 12 cylinder Model 244G turbocharged prime mover. The North Little Rock Shops also installed a custom three-stack exhaust mainfold and replaced Alco's mechanically-driven radiator fan with EMD electrically-driven fans. Almost 600 horsepower was lost in the conversion, however this was not a concern to MP, since the RS3m/GP12 locomotives were to be assigned as yard power. This unit was retired in December 1974 and traded to GE.

In 1973 the Green Bay & Western embarked on a program to upgrade and remanufacture its remaining fleet of locomotives equipped with Alco Model 244 engines – four RS-2’s and four RS-3’s. The approach taken was the brainchild of GB&W’s Norwood Shops Diesel Supervisor Ralph Stuteen. He reviewed a program undertaken by the Portuguese Railways to re-engine their fleet of RSC-2’s and RSC-3’s with Model 251C’s. Since the 12-cylinder Model 251 engine was longer than the 12-cylinder Model 244 due to the 251’s greater width of crankshaft bearings, Stuteen concluded that the slightly shorter long hood of the RS-2 would not permit shoehorning of a Model 251 into it without major modification, and the RS-2’s became recipients of hand-me-down components (from the RS-3’s) in the GB&W program. The re-built RS-3’s were designated RS-20’s due to the 2000 hp produced by the Model 251C. The GB&W’s RS-20 program was highly successful, and attracted the interest of other Alco-committed railroads and of locomotive builder Morrison-Knudsen of Boise, Idaho. Morrison-Knudsen used the GB&W RS-20 program as a template to market the remanufacture of RS-3’s which were still being operated in significant numbers but were otherwise nearing retirement age. The RS-3’s remanufactured were designated as TE 56-4A by M-K, and also incorporated the 12-cylinder Model 251C rated at 2000 horsepower. Additional new major components included radiators and cooling fan, lube oil cooler, an AAR-Type control stand, and 26L air brake equipment. The rear belt-driven traction motor blower was eliminated, and cooling air was ducted from a higher capacity front blower to provide cooling for the rear traction motors. The tip-off to this feature is a flat duct mounted on top of the long hood running board on the locomotive’s left side. In lowering the short hood, this end became the front of the locomotive. The remainder of the major components were rebuilt, and the locomotive was completely rewired. After M-K completed a pair of modified TE 56-4A specification (1800 hp) units for the Detroit & Mackinac on August 20, 1975, the company undertook a far more ambitious TE 56-4A effort, the remanufacture of eight RS-3’s for the D&H. Up to that point, the D&H had been working on a design of its own, and had gone so far as to complete a locomotive superstructure at its Colonie Shops. At that point the project was abandoned in favor of shipping 10 RS-3’s to M-K for TE 56-4A remanufacture. Like almost all of D&H’s large roster of RS-3’s, these units were dynamic brake equipped, a feature the D&H wished to retain on the remanufactured units. The original short hood formerly contained the dynamic brake assembly, and with the short hood being cut down to “low-nose” configuration, the dynamic brake had to be repositioned. So M-K designed a box to contain the resistor grid and blower assembly and mounted it on the top of the long hood above the generator (just behind the cab). To provide room for additional equipment and improved cooling for the 2000 hp Model 251C engine, the long hood was increased in height by 6½ inches. The first D&H unit (#501) was completed on December 13, 1975, followed by three more by the end of the year, and the final four in early 1976. Number 506 came from M-K as #1976, in a Bicentennial red-white-blue adaption of the D&H’s “lightning-stripe” scheme. Remanufactured RS-3’s are typically referred to as RS-3u’s. A number of other railroads performed more modest modifications of RS-3’s, primarily in adopting the low-nose configuration.