The best way to do this imho is to reverse the wiring on the plug that goes to the extruder motor. I say this as the longer you own the printer, the more times you’ll have to remember to account for the reversed extruder.
There’s a good explanation about this here - and with this method, you don’t have to get to the mainboard end by disassembling the case.
As for the builder, if you can provide me with the build id of the run, I can have a look and see what the story is…