[ns] Dynamically change data rate

Hi,
In Tcl if we want to change the CBR value and data rate dynamically then
how it can be achieved?
i have tried following code but it takes only initialisation value that is
1000

set cbr_rate 1000
proc record {} {

global cb_rate cbr

set ns [Simulator instance]

set $cb_rate [expr $cb_rate + 50000]
set time 0.5
set now [$ns now]
$cbr set rate_ $cb_rate
puts "$cb_rate"
$ns at [expr $now+$time] "record"

}

Thanks in advance
--
Regards