I don't think you can. Make sure to move all your troops out of the capital so your rubbish little militia army doesn't have too much trouble.

Don't know if you know but to get to constitutional monarchy from absolute monarchy you've got to get your lower classes to revolt. After they win you'll be a republic. Then you've got to make your upper classes revolt to become a constitutional monarchy. It's a bit of a round about way but sort of make sense I suppose.