[{_id:"bmasndvhjbcw",name:"lucas",occupation:"scientist",passed_phd:true,age:55,location:"texas",},{_id:"bmasndvhjbcx",name:"mark",occupation:"scientist",age:45,passed_phd:true,location:"texas",},{_id:"bmasndvhjbca",name:"stuart",occupation:"lab assistant",age:25,passed_phd:false,location:"texas",},{_id:"bmasndvhjbcq",name:"cooper",occupation:"physicist",age:69,passed_phd:false,location:"texas"}]

db.collection.aggregate([{$addFields:{passed_phd:{$toString:"$passed_phd"}}}])