Database
[{"A":1,"B":2,"":3,"D":4}]
Query
db.collection.update({},[{$project:{docArray:{$map:{input:{$objectToArray:"$$ROOT"},in:{$cond:{if:{$regexMatch:{input:"$$this.k",regex:"[A-Za-z0-9]"}},then:"$$this",else:{k:"C",v:"$$this.v"}}}}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$docArray"}}}])