Blender Version
v2.69.11 f1a0278

Short description of error
In Sequencer, if you set a Proxy Custom File to a movie strip and then want to change it (maybe because you've selected the wrong file), the proxy will not be updated until reopen file or restart Blender. Reset proxy checkboxes or "Refresh Sequencer" don't help.

Steps to reproduce the error

1. Add a movie/image strip to sequencer
2. Enable Proxy, Proxy Custom File and select a movie/image to use as Proxy.
3. Change Proxy display settings to check if the Proxy loaded properly.
4. Now choose another file to use as Proxy.
5. Proxy will not update until reopen file or restart Blender.