S4; Setter function is not chaning slot value as expected

It is my first time programming with S4 and I can't get the setter fuction
to actually change the value of the slot created by the constructor.

I guess it has to do with local copy, global copy, etc. of the variable -
but, I could't find anything relevant in documentation.

Tried to copy examples from the internet, but they had the same problem.

# The code
     setClass ("Account" ,
                representation (
                customer_id = "character",
                transactions = "matrix")
     )


     Account <- function(id, t) {
             new("Account", customer_id = id, transactions = t)
             }


     setGeneric ("CustomerID<-", function(obj,
id){standardGeneric("CustomerID<-")})

     setReplaceMethod("CustomerID", "Account", function(obj, id){
         obj at customer_id <- id
         obj
         })

         ac <- Account("12345", matrix(c(1,2,3,4,5,6), ncol=2))
         ac
         CustomerID <- "54321"
         ac

#Output

         > ac

         An object of class "Account"
         Slot "customer_id":
         [1] "12345"

         Slot "transactions":
              [,1] [,2]
         [1,]    1    4
         [2,]    2    5
         [3,]    3    6

# CustomerID is value has changed to 54321, but as you can see it does't

         > CustomerID <- "54321"

         > ac

         An object of class "Account"
         Slot "customer_id":
         [1] "12345"

         Slot "transactions":
              [,1] [,2]
         [1,]    1    4
         [2,]    2    5
         [3,]    3    6


Help!

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