How to change framerate dynamically

[fb8fb8 at gmail.com]

Hi,

I can use gstreamer to stream a mpg video from one Linux PC to another. 
The framerate of the video can be changed by this command:

gst-launch filesrc "location= 1.MPG" ! decodebin ! videorate ! video/x-raw-yuv, width=640,height=480,framerate=15/1 ! ffmpegcolorspace ! jpegenc ! multipartmux ! tcpserversink host=192.168.2.1 port=5000
In this command, framerate is set to be 15. The video is streamed at framerate 15 from beginning to end.
But I want to change framerate dynamically, which means during the streaming I can change framerate anytime. After changing framerate, the video will continue but stream at the new framerate.

How can I change framerate in this way?
Thanks!
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