how to dynamically change video frame rate

[Tom]

I use
gst-launch filesrc "location=video/1.MPG" ! decodebin ! videorate !
video/x-raw-yuv, width=640,height=480,framerate=25/1 ! ffmpegcolorspace !
jpegenc ! multipartmux ! tcpserversink host=192.168.2.1 port=$portNumber
to stream a video and can change the video frame rate in this way.

But in this way the frame rate is fixed (i.e., framerate=25/1 here).
How can I change the frame rate dynamically?
I mean, during the streaming of this video, I can change the framerate
anytime I want.

Thanks!
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