Change connection interval timming

Hi. Dialog
I want to change connection interval in real time.
I've already tested "GAPC_PARAM_UPDATE_CMD" in app_param_update_start().
它在fi工作rst time called in "app_connection_func" app_proxr_proj.c .
But in conected state, I want to re-change another interval, so re-call that func(app_param_update_start), It works none.
My Function is below.

extern bool connect_fast_speed;
void app_param_update_func(void)
{
struct gapc_param_update_cmd * req = KE_MSG_ALLOC(GAPC_PARAM_UPDATE_CMD, TASK_GAPC, TASK_APP, gapc_param_update_cmd);
// Fill in the parameter structure
req->operation = GAPC_UPDATE_PARAMS;

if(connect_fast_speed==1)
{
req->params.intv_min = 50;
req->params.intv_max = 80;
req->params.latency = 0;
req->params.time_out = 800;
}
else
{
req->params.intv_min = 160; // N * 1.25ms
req->params.intv_max = 200; // N * 1.25ms
req->params.latency = 0; // Conn Events skipped
req->params.time_out = 800; // N * 10ms
}

ke_msg_send(req);
return;
}

So i'm very confused this situation.
Why this func. works just one time?

Thanks