Excellent question. Unfortunately, there is no straight answer. While 8 months is less than half of your sentence, you do have a lot going for you by way of evidence that you've turned you life around. In addition, you're not asking to terminate your probation, but rather convert your community control to additional probation time. My opinion is that this is a motion worth filing. You could entitle it "Defendant's Motion to Terminate Community Control and Convert Balance to Probation." A lot will depend on your lawyer's ability (or your ability if you go it alone) to convince the judge that you are worth taking a risk on. As you undoubtably agreed to 2 years CC in your plea agreement, the judge is under no obligation to let you remove the CC early.
There is a lot more that I wish I could write out but it's a lot simpler just to talk in person.
Mr. Wolenski is correct. You can file a motion to convert your community control at any time but you are right that your chances increase significantly once you reach that 50% mark.