If the *entire* run of wire from that receptacle back to the breaker
is rated for 20A (12 guage) then you could replace the existing 15A
receptacle with one that would mate with the 20A plug. If any/all
of that wire is 15A (14 guage) then it's unsafe and against code.
The 20A plug is there for a reason. Whatever the machine is, it
likely draws >15A either constantly or maybe during startup.
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