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Blue Boxes actually adapt to DCC very well. The original sintered iron wheelsets don't pick up current as well as nickle-silver and so some folks change them out to NWSL wheelsets made specifically for retrofitting on Athearn locos. I change out every other one so that I have the traction of the sintered iron and the conductivity of the nickle-silver.
The only problem they may have been refering to is the first and second generation motors of the early 60s thru the early 80s. These motors have a larger amp draw and so tend to fry decoders. The newer 3rd generation motors are skew wound and have a much better, much lower amp draw. 3rd generation motors can be swapped with 2nd generation motors quite easily and are available regularly on Ebay. 1st generation Globe motors are not so easily changed over but are not easy to come by either.
A-Line and other manufacturers offer remotor kits but those can get pricey and defeat the purpose that Athearns were designed for---a low cost, good running, easy to maintain loco to introduce many to the hobby.
When buying one just look up thru the frame from underneath. If it has brass flywheels, it would be the newer motor. Looking at the power trucks and seeing brass or bronze bearings and plastic sideframes is another way to tell.