Legal Question in Business Law in California

I'm not sure you can do that at all this close to the trial date, let alone ex parte. That's something I'd need to research before I could give you actual advice. But even if the procedure is still available, the judge might deny your request. Based on the very limited information you've offered, the reasons to say no seem stronger than the reasons to say yes. Of course, it's very possible that I would see things differently if I knew more about the case.

The defendant has a due process right to prepare his case based on what you've alleged, and one of the things you've alleged is that your damages were no more than $25,000. What's more, had this been an unlimited-jurisdiction case all along, he (and you) would have been allowed to conduct more discovery and would have had other options that aren't available in a limited-jurisdiction case. He also might have opted to hire a lawyer instead of representing himself if he'd known the claim was for more than $25,000. The change you propose would expose him to a larger judgment after denying him the opportunity to prepare a more thorough defense.

I'd be more sympathetic to you if the trial date wasn't so close. But even then, you would need to explain why you filed a smaller case to begin with. By itself, the fact that you are only now learning the extent of your damages is not a very good reason to give you the relief you want, though it will depend on why you didn't know sooner. (If it's somehow the defendant's fault, then you will have a much better argument.)

If you had thought of this sooner and brought a regular motion, your pro per opponent would have had time to come up with arguments to oppose it. By going in ex parte, you're asking the court to rule on your motion before the defendant can even oppose it properly.

Let's say the judge is allowed to grant your application and decides to do so. She will most likely delay the trial, reopen discovery for the defendant (but perhaps not for you), and take other steps to protect his due process rights. Some of those steps might be costly for you. For example, if you're making his defense more expensive than it should have been, she might order you to cover those expenses.

If you do apply ex parte to convert the case, your application will need both to explain why converting the case would serve the interests of justice and to justify making the request in a way that denies the defendant the usual amount of advance notice. That means you will need to show that your mistake about damages was reasonable, that you could not reasonably have figured it out much sooner than you did, and that you then brought the application promptly. You will also need to persuade the court that your interest in converting the case outweighs the defendant's interests in proceeding as a limited-jurisdiction case. Remember, you're the one who decided to make it that type of case to begin with. By itself, saying you made the wrong decision won't be nearly enough.