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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