Dave's right, I misread your question and thought you required that the url does not change. So there are two options: 1) simple, when the url can change use ad_returnredirect and ad_script_abort like in hundreds of places all over the toolkit 2) if the url should not change, use rp_internal_redirect according to Randy's example.
One correction to Randy's example: you should use abstract urls for the target, e.g. say
rp_internal_redirect "anotherpage"
instead of
rp_internal_redirect "anotherpage.tcl"