Modifying timezone of a POSIXct object without changing the display

Question

I have a POSIXct object and would like to change it's tz attribute WITHOUT R to interpret it (interpret it would mean to change how the datetime is displayed on the screen).

Some background: I am using the fasttime package from S.Urbanek, which take strings and cast it to POSIXct very quickly. Problem is that the string should represent a datetime in "GMT" and it's not the case of my data.

I end up with a POSIXct object with tz=GMT, in reality it is tz=GMT+1, if I change the timezone with

attr(datetime, "tzone") <- "Europe/Paris";
datetime  <- .POSIXct(datetime,tz="Europe/Paris"); 

then it will be "displayed" as GMT+2 (the underlying value never change).

EDIT: Here is an example

datetime=as.POSIXct("2011-01-01 12:32:23.234",tz="GMT")
attributes(datetime)
#$tzone
#[1] "GMT"
datetime
#[1] "2011-01-01 12:32:23.233 GMT"

How can I change this attribute without R to interpret it aka how can I change tzone and still have datetime displayed as "2011-01-01 12:32:23.233" ?

EDIT/SOLUTION, @GSee's solution is reasonably fast, lubridate::force_tz very slow

datetime=rep(as.POSIXct("2011-01-01 12:32:23.234",tz="GMT"),1e5)
f <- function(x,tz) return(as.POSIXct(as.numeric(x), origin="1970-01-01", tz=tz))
> system.time(datetime2 <- f(datetime,"Europe/Paris"))
   user  system elapsed 
   0.01    0.00    0.02 
> system.time(datetime3 <- force_tz(datetime,"Europe/Paris"))
   user  system elapsed 
   5.94    0.02    5.98 
identical(datetime2,datetime3)
[1] TRUE

Answer 1

To change the tz attribute of a POSIXct variable it is not best practice to convert to character or numeric and then back to POSIXct. Instead you could use the force_tz function of the lubridate package

library(lubridate)

datetime2 <- force_tz(datetime, tzone = "CET")
datetime2
attributes(datetime2)