I know just how you feel about changing jobs.I recently left the company that i had worked at for 17 years.It was a hard decision but the company had gone from being one of the best in the business to one of the worst to work for and i felt i needed to leave the sinking ship or drown with it.I now make more money than i would have ever made with them and enjoy the professional atmosphere that i work in now.
I would ask for the bonus since by promising it they made a verbal contract with you.I don't think you should give it to yourself without permission though because you will be opening yourself up to charges of fraud and embezzelment.If they are that petty to you because of leaving they might just jump at the chance to try to hurt you.
If they need you to keep helping out they will honor their commitment that they made as far as the bonus goes or then they could do without my services if i was in your shoes.
GOOD LUCK AND I HOPE YOU ENJOY YOUR NEW JOB!!