[R] change the class of an object within with()

[François MICHONNEAU]

Hello,

Can anyone explain me what is wrong in the following code? and in particular why it is not 
possible to change the class of an object thanks to the function with(). Does an 
alternative exist?

xxx <- data.frame(x = c("a","b","c","d","e"), y = c("1","2","3","4","5"))
str(xxx)
with(xxx, {
	x <- as.character(x)
	y <- as.numeric(y)
})
str(xxx) #no effect on the class of x and y

xxx$x <- as.character(xxx$x)
xxx$y <- as.numeric(xxx$y)
str(xxx)

Thanks

François Michonnneau