Update the key data type from string to int64

Working with Data
,
7 
db.RetailerRequest.updateMany
(
   {"fulfill.tagSerialNumbers":{$type:"string"}},
   [
   	{$set:
   		{fulfill:
   			{$map:
   			{input:"$fulfill",
   			in:{$mergeObjects:
   			["$this",{tagSerialNumbers:
   			{ $toLong: "$$this"}}]}}}
   		}
   	}
   ] 
)

Now i am getting the error: MongoServerError: Unsupported conversion from object to long in $convert with no onError value

Please let me know how to proceed further