I have gone through the process of changing a 5F to a 4C. I was with the Brocks, went NES and was released. Two or three years later, I went back to college in Barrie and thought about getting back into the reserves to supplement my meager income. Long story short, I went with a bunch of papers I had from when I was in before, showed them to a WO there at the armouries, who then informed me that I was Dishonourably Discharged, and since I didn't know that, I should contact my MP, as I may have a legal case against the government....So I did. A Ministerial Inquiry was initiated, an investigation begun into the circumstances of my release, and after a while, the Commander, LFCA directed my release item be changed to 4C , with a caveat that if I go NES I would be released 5F with no chance of it being reversed again.
I ended up joining the Comm Res in Borden, and 4 years later CTed to the Reg force, and here I am today.
I still have all the paperwork....just in case!
Good luck, it can be done!