I was attempting to do exactly this when going through existing amenity=office and spotted that large chunk of them is just duplication of office= where bothering mappers seems to not be a good use of time - just editing objects to drop amenity=office is better.
Then it turned out that there is sufficiently many of such objects to run it as a bot edit.
Because it will not be replaced, it will be removed where it already duplicates office= tagging (and not edited otherwise)
Where office= has one of expected values. Current list is listed below, but it may be changed in future. Cases where office has no valid value, say amenity=office office=gibberish262663636 will not be edited at all.
accountant, advertising_agency, architect, association, chamber, charity, company, construction_company, consulting, courier, coworking, diplomatic, educational_institution, employment_agency, energy_supplier, engineer, estate_agent, financial, financial_advisor, forestry, foundation, geodesist, government, graphic_design, guide, harbour_master, insurance, it, lawyer, logistics, moving_company, newspaper, ngo, notary, politician, political_party, property_management, publisher, quango, religion, research, security, surveyor, tax_advisor, telecommunication, therapist, travel_agent, tutoring, union, university, visa, water_utility, healthcare, yes