Database
db={foos:[{"values":[{"foo":"12"},{"foo":"34"},{"foo":"56"}]}]}
Query
db.foos.update({"values":{"$type":"array"}},[{"$set":{"values":{"$map":{"input":"$values","in":{"$cond":{"if":{"$eq":[{"$type":"$$this.foo"},"string"]},"then":{"$mergeObjects":["$$this",{"foo":{"$toDecimal":{"$trim":{"input":"$$this.foo"}}}}]},"else":"$$this"}}}}}}])