I've done that before. If you can't void the bet, if the line hasn't moved, just bet the otherside up to the point that you wanted to bet. You lose a little a juice, but not a ton.
Also, in most instances, the cash out option is not as good as hedging by taking the other side (or other sides), particularly if you have the favorite. For example, if you have a chance to hit a parlay with one leg left out; rather than cash out, you can do better by taking the other side in the remaining parlay leg. If the team you have in the parlay is the favorite on the ML, take the dog with the points and try to hit the middle, that almost always result in a more favorable payout regardless of outcome than cashing out. Run the numbers for all of the betting options before cashing out, there is a reason the books offer a cash out, the percentages are in their favor.