Let's say I set a bounty for 50, and I don't any answer at all. Will I be able to change the bounty value from 50 to 100?
Once the grace period for the first bounty ends, you will be able to start a new bounty, if you don't have already three open bounties. As it would be the second bounty you offer for the same question, your second bounty needs to be higher than 50.
Once the bounty has been started, you cannot change its value anymore. In the case there aren't answers that gain the bounty (automatically, or manually), you don't get your reputation back.