I had a 5F release changed to a 4C, but it took about 2 years beginning with a call to my MP. A Ministerial Inquiry was in the works, but it was decided that it was not necessary. eventually the Militia district (In know, I'm dating myself) said it would not change my release, but would let me back in. Some General ordered them to change it from 5F to 4C, and it was done.
That was some time ago, '97 it started and I was back in the Reserves by spring '99. I'll see if I can find any of the paperwork I have on it for more details.