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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